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/w6c/6.out.h

@@ -40,6 +40,7 @@ enum {
 	A_NOP = 0,
 	A_TEXT,
 	A_DATA,
+	A_DATAW,	/* writable DATA: lands in .data (RW) instead of .text */
 	A_GLOBL,
 	A_END,