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/6l/obj.c

@@ -52,8 +52,8 @@ typedef struct {
 #define ELF64_ST_TYPE(i)	((i) & 0xf)
 #define ELF64_ST_BIND(i)	((i) >> 4)
 
-static int
-read_all(const char *path, u8 **out, u64 *len)
+int
+l_read_all(const char *path, u8 **out, u64 *len)
 {
 	FILE *f = fopen(path, "rb");
 	if (f == NULL) return -1;
@@ -69,6 +69,8 @@ read_all(const char *path, u8 **out, u64 *len)
 	return 0;
 }
 
+#define read_all l_read_all
+
 static void
 emit_text(Lnk *l, const u8 *src, u64 n)
 {
@@ -229,6 +231,16 @@ l_load(Lnk *l, const char *path)
 	if (read_all(path, &buf, &len) < 0) return -1;
 	if (len >= 8 && memcmp(buf, "!<arch>\n", 8) == 0)
 		return load_archive(l, path, buf, len);
+	/* Shared object: dispatch to dyn.c, which re-reads (small loss
+	 * for a much cleaner separation of static vs dynamic loaders). */
+	if (len >= sizeof(Ehdr)) {
+		Ehdr *eh = (Ehdr *)buf;
+		if (memcmp(eh->e_ident, "\x7f""ELF", 4) == 0
+		    && eh->e_type == 3 /* ET_DYN */) {
+			free(buf);
+			return l_load_so(l, path);
+		}
+	}
 	return load_image(l, path, buf, len);
 }