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/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,