w6l+selfhost: dynamic-link + .data — shared R+W segment

.data now lives at the end of the dyn-path R+W PT_LOAD, just after
.dynamic. The single segment covers .got.plt + .dynamic + .data; its
filesz drops trailing zeros (BSS) while memsz spans the full extent.

Relocation moves from main.c into each emit function so the static
and dynamic paths use their own data_va — text→data refs land on the
right VA regardless of path. Removes the early-error in dynout.c
that previously refused any .data with -l/-L.

Tests: 810_dyn gains two new dyn+.data fixtures (mutable read+write
of an i32, plus a zero-init i64 verifying the BSS scan still produces
a valid p_filesz<p_memsz under the shared segment).

cmd/w6l/out.c

@@ -63,6 +63,16 @@ l_emit_elf(Lnk *l, FILE *f, u64 base, u64 entry)
 	const u64 rx_end   = text_off + l->textlen;
 	const int has_data = (l->datalen > 0);
 
+	/* data goes at the next page boundary so the loader can grant a
+	 * fresh page of R+W permissions without overlapping the R+X mapping. */
+	const u64 data_off = has_data ? ((rx_end + page - 1) & ~(page - 1)) : 0;
+	const u64 data_va  = has_data ? (base + data_off) : 0;
+
+	/* Apply relocations now that the layout's text_va/data_va are
+	 * known. Deferred from main.c so the dynamic-link path can use
+	 * its own data_va. */
+	if (l_relocate(l, base + text_off, data_va) != 0) return 1;
+
 	/* BSS optimisation: trailing zero bytes in .data can be left out
 	 * of the file. The loader zero-fills the gap between p_filesz and
 	 * p_memsz, so this shrinks the binary without changing semantics.
@@ -75,11 +85,6 @@ l_emit_elf(Lnk *l, FILE *f, u64 base, u64 entry)
 			bsslen++;
 	}
 	const u64 data_file_len = l->datalen - bsslen;
-
-	/* data goes at the next page boundary so the loader can grant a
-	 * fresh page of R+W permissions without overlapping the R+X mapping. */
-	const u64 data_off = has_data ? ((rx_end + page - 1) & ~(page - 1)) : 0;
-	const u64 data_va  = has_data ? (base + data_off) : 0;
 	const u64 file_end = has_data ? (data_off + data_file_len) : rx_end;
 	(void)data_va;