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).

Makefile

@@ -34,7 +34,8 @@ C6_OBJ	= $(C6_SRC:cmd/6c/%.c=$(OBJ)/6c/%.o)
 A6_SRC	= cmd/6a/main.c cmd/6a/lex.c cmd/6a/parse.c cmd/6a/asm.c cmd/6a/obj.c
 A6_OBJ	= $(A6_SRC:cmd/6a/%.c=$(OBJ)/6a/%.o)
 
-L6_SRC	= cmd/6l/main.c cmd/6l/obj.c cmd/6l/sym.c cmd/6l/pass.c cmd/6l/out.c
+L6_SRC	= cmd/6l/main.c cmd/6l/obj.c cmd/6l/sym.c cmd/6l/pass.c cmd/6l/out.c \
+	  cmd/6l/dyn.c cmd/6l/dynout.c
 L6_OBJ	= $(L6_SRC:cmd/6l/%.c=$(OBJ)/6l/%.o)
 
 RT_S	= rt/start.s rt/syscall.s rt/alloc.s rt/streq.s rt/abort.s
@@ -170,8 +171,8 @@ $(BIN) $(LIB) $(OBJ)/wwc $(OBJ)/ww $(OBJ)/wwdump $(OBJ)/6c $(OBJ)/6a $(OBJ)/6l $
 # Each phase adds a $(BIN)/test_<name> target; the runner walks them.
 TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_6c $(BIN)/test_6a $(BIN)/test_6l $(BIN)/test_arch \
-	  $(BIN)/test_e2e $(BIN)/test_ffi $(BIN)/test_stdlib $(BIN)/test_selfhost \
-	  $(BIN)/test_6a_ww $(BIN)/test_6l_ww $(BIN)/test_ww_ww
+	  $(BIN)/test_e2e $(BIN)/test_ffi $(BIN)/test_dyn $(BIN)/test_stdlib \
+	  $(BIN)/test_selfhost $(BIN)/test_6a_ww $(BIN)/test_6l_ww $(BIN)/test_ww_ww
 
 $(BIN)/test_smoke: test/wwc/000_smoke.c $(LIB)/libwwc.a | $(BIN)
 	$(CC) $(CFLAGS) $(INCS) -o $@ $< -L$(LIB) -lwwc
@@ -205,6 +206,10 @@ $(BIN)/test_e2e: test/wwc/700_e2e.c $(BIN)/ww $(BIN)/6c $(BIN)/6a $(BIN)/6l \
 $(BIN)/test_ffi: test/wwc/800_ffi.c $(BIN)/6c | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+$(BIN)/test_dyn: test/wwc/810_dyn.c $(BIN)/ww $(BIN)/6c $(BIN)/6a $(BIN)/6l \
+		$(LIB)/libwwrt.a | $(BIN)
+	$(CC) $(CFLAGS) -o $@ $<
+
 $(BIN)/test_stdlib: test/wwc/900_stdlib.c $(BIN)/6c | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<