w6a: DATAW directive for writable .data section

First step toward top-level mutable `let`. Adds a sibling directive to
DATA whose bytes land in a separate writable .data PROGBITS section
(SHF_ALLOC|SHF_WRITE, STT_OBJECT) instead of .text. The section is
emitted only when DATAW was used, so inputs without it produce a
byte-identical .o — tests 991 (selfhost .o diff) and 995 (self-rebuild)
keep passing unchanged.

w6l still treats data-resident syms as undefined; that's the next step.

cmd/w6a/asm.c

@@ -54,6 +54,17 @@ a_emit_u32(Asm *a, u32 v)
 	a_emit_byte(a, (u8)((v >> 24) & 0xff));
 }
 
+void
+a_emit_data_byte(Asm *a, u8 b)
+{
+	if (a->datalen + 1 > a->datacap) {
+		u64 nc = a->datacap ? a->datacap * 2 : 256;
+		a->data = realloc(a->data, nc);
+		a->datacap = nc;
+	}
+	a->data[a->datalen++] = b;
+}
+
 void
 a_addreloc(Asm *a, u64 off, int kind, Asym *s, i64 add)
 {
@@ -310,6 +321,20 @@ a_encode(Asm *a)
 				a_emit_byte(a, p->bytes[i]);
 			break;
 		}
+		case A_DATAW: {
+			/* Writable variant: bytes go into .data (RW) instead
+			 * of .text. obj.c emits the extra section conditionally
+			 * on datalen > 0 so .o output stays byte-identical for
+			 * inputs that don't use DATAW. */
+			Asym *s = a_intern(a, p->to.sym);
+			s->defined = 1;
+			s->is_data = 1;
+			s->is_global = 1;
+			s->addr = a->datalen;
+			for (u64 i = 0; i < p->nbytes; i++)
+				a_emit_data_byte(a, p->bytes[i]);
+			break;
+		}
 		case A_RET:
 			a_emit_byte(a, 0xC3);
 			break;