w6c+selfhost: codegen for top-level mutable let

Third step toward writable globals. The C cgen and its selfhost
mirror now:

  - emit DATAW <name>(SB),"<8 LE bytes>" for every top-level `let`
    whose type lands in the scalar set (i8..i64/u8..u64/bool/rune/
    int/uint/uintptr/ptr; floats and multi-word types deferred);
  - drop the "no writable .data" silent-drop guard at the N_IDENT
    store path, replacing it with a RIP-relative MOVQ for `=` and
    a load→combine→store sequence for the compound ops; and
  - route `&name` through LEAQ name(SB) instead of dropping it.

Type aliases resolve via aliaslookup so `type counter = i32; let c:
counter = 0;` still emits a DATAW slot. Non-literal initialisers
silently skip, which surfaces as a clean undefined-symbol error if
the binding is ever referenced.

The selfhost mirror lands in the same commit because test 990
diffs the C cgen against wwdump_ww -c on err.ww (which has
top-level `let nerrors: i32 = 0; ... nerrors += 1;`). Any drift
between the two cgens makes 990 fail. Bootstrap stays at a fixed
point: ww2 == ww3 == ww4 byte-identical.

cmd/w6c/txt.c

@@ -25,6 +25,7 @@ anames(int op)
 	case A_NOP:	return "NOP";
 	case A_TEXT:	return "TEXT";
 	case A_DATA:	return "DATA";
+	case A_DATAW:	return "DATAW";
 	case A_GLOBL:	return "GLOBL";
 	case A_END:	return "END";
 	case A_MOVQ:	return "MOVQ";