6l: dynamic linking with symbol versioning

Teach the linker to consume ET_DYN shared objects and emit a
dynamically-linked ELF executable. Snake et al. can now link
against libncurses + libc through the system dynamic loader.

Pipeline additions:

- dyn.c: read ET_DYN, parse .dynsym + DT_SONAME, walk
  .gnu.version_d / .gnu.version to learn each export's default
  version (skip hidden entries).
- pass.c: when an undefined sym is provided by some Lso,
  promote it to dynamic, assign a PLT slot, record the
  matched version on the Lsym.
- dynout.c: emit PT_INTERP + PT_DYNAMIC, .dynsym/.dynstr/.hash,
  .plt + .got.plt + .rela.plt, .gnu.version + .gnu.version_r,
  and the full DT_* set with DT_BIND_NOW. Patch PC32/PLT32
  references against dyn syms to point at their PLT stubs.
- main.c: -L<dir> and -l<name> flag parsing; resolve <name>
  via .so / .so.<N> / .a in libdir order, skipping GNU ld
  linker scripts (libc.so on most distros).
- ww driver: collect -l/-L (joined and split forms) and pass
  through to 6l.

Design choices:

- DT_BIND_NOW so the loader resolves all PLT slots at startup;
  no PLT0 lazy resolver stub.
- SysV .hash, not .gnu.hash. One bucket; loader scans the
  chain. Slow at scale, fine for snake-class binaries.
- Non-PIE at fixed 0x400000.
- No section headers — loader uses program headers, but
  readelf -V/-S won't display anything.

Symbol versioning is the only correctness item beyond the
basic PLT/GOT machinery: glibc symbols default to versions
later than GLIBC_2.2.5 (e.g. clock_gettime → GLIBC_2.17 for
the vDSO impl), and the loader rejects unversioned references
to those without a matching Vernaux entry.

test/wwc/810_dyn covers four cases: bare libc dyn call,
multi-PLT, clock_gettime versioning, and fn-pointer to FFI
binding (which exercises the codegen fixes from the parent
commit alongside the new linker path).

cmd/ww/main.c

@@ -156,7 +156,8 @@ expand(FILE *out, const char *path, struct ImportSet *visited,
 }
 
 static int
-build_one(const char *src, const char *out, const char *extra_includes)
+build_one(const char *src, const char *out, const char *extra_includes,
+    const char *extra_libs, const char *extra_libdirs)
 {
 	const char *c6 = toolpath("WW_6C", "6c");
 	const char *a6 = toolpath("WW_6A", "6a");
@@ -237,7 +238,13 @@ build_one(const char *src, const char *out, const char *extra_includes)
 		snprintf(a2, sizeof a2, "%s/../obj/rt/syscall.o", self_dir);
 		snprintf(rtargs, sizeof rtargs, "%s %s", a1, a2);
 	}
-	snprintf(cmd, sizeof cmd, "%s -o %s %s %s", l6, out, obj, rtargs);
+	/* -L<dir> goes before -l<name> so 6l can resolve the latter. */
+	const char *libargs   = (extra_libs    && extra_libs[0])    ? extra_libs    : "";
+	const char *libdirset = (extra_libdirs && extra_libdirs[0]) ? extra_libdirs : "";
+	snprintf(cmd, sizeof cmd, "%s -o %s %s %s%s%s%s%s",
+	    l6, out, obj, rtargs,
+	    libdirset[0] ? " " : "", libdirset,
+	    libargs[0]   ? " " : "", libargs);
 	if (run(cmd) != 0) {
 		fprintf(stderr, "ww: 6l failed\n");
 		return 1;
@@ -256,21 +263,26 @@ static int
 do_build(int argc, char **argv)
 {
 	const char *src = NULL;
-	char libs[2048] = {0};
-	char incs[2048] = {0};
+	char libs[2048]    = {0};	/* -l<name> entries, space-separated */
+	char libdirs[2048] = {0};	/* -L<dir>  entries, space-separated */
+	char incs[2048]    = {0};
 	for (int i = 0; i < argc; i++) {
 		if (strncmp(argv[i], "-l", 2) == 0 && argv[i][2]) {
-			char libpath[512];
-			const char *libdir = getenv("WW_LIB");
-			if (libdir == NULL) {
-				static char def[1024];
-				snprintf(def, sizeof def, "%s/../lib", self_dir);
-				libdir = def;
-			}
-			snprintf(libpath, sizeof libpath, "%s/lib%s.a",
-			    libdir, argv[i] + 2);
 			size_t n = strlen(libs);
-			snprintf(libs + n, sizeof libs - n, " %s", libpath);
+			snprintf(libs + n, sizeof libs - n,
+			    "%s%s", n ? " " : "", argv[i]);
+		} else if (strcmp(argv[i], "-l") == 0 && i + 1 < argc) {
+			size_t n = strlen(libs);
+			snprintf(libs + n, sizeof libs - n,
+			    "%s-l%s", n ? " " : "", argv[++i]);
+		} else if (strcmp(argv[i], "-L") == 0 && i + 1 < argc) {
+			size_t n = strlen(libdirs);
+			snprintf(libdirs + n, sizeof libdirs - n,
+			    "%s-L%s", n ? " " : "", argv[++i]);
+		} else if (strncmp(argv[i], "-L", 2) == 0 && argv[i][2]) {
+			size_t n = strlen(libdirs);
+			snprintf(libdirs + n, sizeof libdirs - n,
+			    "%s%s", n ? " " : "", argv[i]);
 		} else if (strcmp(argv[i], "-I") == 0 && i + 1 < argc) {
 			size_t n = strlen(incs);
 			snprintf(incs + n, sizeof incs - n,
@@ -290,10 +302,7 @@ do_build(int argc, char **argv)
 	snprintf(out, sizeof out, "%s", base);
 	char *dot = strrchr(out, '.');
 	if (dot && strcmp(dot, ".ww") == 0) *dot = '\0';
-	(void)libs;	/* libs string is gathered; build_one currently
-			 * always links libwwrt.a; -l support pending more
-			 * glue between driver and 6l invocation. */
-	return build_one(src, out, incs);
+	return build_one(src, out, incs, libs, libdirs);
 }
 
 static int
@@ -302,7 +311,7 @@ do_run(int argc, char **argv)
 	if (argc < 1) { fputs("ww run: missing source\n", stderr); return 2; }
 	char tmp[1024];
 	snprintf(tmp, sizeof tmp, "/tmp/ww_run_%d", getpid());
-	if (build_one(argv[0], tmp, "") != 0) return 1;
+	if (build_one(argv[0], tmp, "", "", "") != 0) return 1;
 	int rc = run(tmp);
 	unlink(tmp);
 	return rc;