w6a: branch labels stay out of the symtab (isglobal wired)

Every interned symbol -- including branch labels defined via
p.label -- was emitted STB_GLOBAL, so labels were collide-able
across objects and the header's "GLOBAL symbols only" contract was
enforced nowhere (asym.isglobal was set but never read). Labels
stay interned for fixup resolution; the symtab now carries only
exported definitions, undefined externs, and reloc-referenced syms
(a reloc pre-pass marks those). rt/ensure.o drops from 15 emitted
syms to 2. Bootstrap 991-995 hold byte-identical on the new format.

selfhost/cmd/w6a/obj.ww

@@ -187,9 +187,23 @@ export fn emitelf(a: *asm_, fd: i32) i32 = {
 	for (zi < 24) { zsym[zi] = 0u8; zi += 1; };
 	bufputb(&sym, zsym.ptr, 24u64);
 
+	// Branch labels are interned for fixup resolution but are NOT
+	// link-visible: emitting them (all bindings here are GLOBAL)
+	// made every label collide-able across objects. Only exported
+	// definitions (isglobal), undefined externs, and reloc-
+	// referenced syms reach the symtab.
+	let rr: *areloc = a.relocs;
+	for (rr != nil) {
+		rr.asy.hasreloc = 1;
+		rr = rr.rnext;
+	};
 	let idx: i32 = 1;
 	let s: *asym = a.syms;
 	for (s != nil) {
+		if (s.defined != 0 && s.isglobal == 0 && s.hasreloc == 0) {
+			s = s.snext;
+			continue;
+		};
 		let entry: [24]u8;
 		let ei: i32 = 0;
 		for (ei < 24) { entry[ei] = 0u8; ei += 1; };

selfhost/cmd/w6a/opcodes.ww

@@ -170,6 +170,7 @@ type asym = struct {
 	istext:    i32,
 	isdata:    i32,	// mutually exclusive with istext; DATAW symbols
 	isglobal:  i32,
+	hasreloc:  i32,	// referenced by a relocation — must be in the symtab
 	addr:      u64,	// offset within its section (.text or .data)
 	idx:       i32,
 	snext:     *asym,