w6a+w6l: DATAR directive for absolute-address relocs in .data

Unblock literal initialisers for str/slice/struct globals by wiring
an R_X86_64_64 relocation kind through both assembler and static
linker.

w6a:
  - new A_DATAR directive, syntax `DATAR slot+off(SB),target(SB)`,
    records an R_X86_64_64 reloc at slot+off in .data pointing at
    target. The slot must be pre-defined by a prior DATAW;
  - parse_operand learned the `name+disp(SB)` shape so the slot's
    byte offset can be addressed explicitly;
  - Areloc carries a `section` flag (0=.text / 1=.data) and obj.c
    splits the reloc list into .rela.text and .rela.data, emitting
    the latter conditionally with sh_info pointing at .data.

w6l:
  - Lrel grows the same `section` flag; obj.c loads `.rela.data`
    sections into the global reloc list with offsets shifted by
    each input's data_off;
  - pass.c handles R_X86_64_64: target VA is data_va+sym.val for
    in_data symbols (else text_va+sym.val), addend is added, and
    the 8-byte slot is patched in l->data (or l->text).

Inputs without DATAR are unaffected — bootstrap, 991 (selfhost .o
diff) and 992 (selfhost exe diff) keep their byte-identical
output. 520_datar covers the new path: asm a DATAW+DATAR pair,
verify .rela.data has exactly one R_X86_64_64 entry, link, run,
confirm the relocated pointer feeds a 5-byte write that prints
"hello".

Selfhost mirror + w6c emission for str/slice/struct literal init
land in follow-ups.

cmd/w6c/6.out.h

@@ -41,6 +41,9 @@ enum {
 	A_TEXT,
 	A_DATA,
 	A_DATAW,	/* writable DATA: lands in .data (RW) instead of .text */
+	A_DATAR,	/* reloc-only: patch a 64-bit slot in .data with a
+			 * symbol's runtime VA. Pairs with a prior DATAW
+			 * that left zero placeholder bytes. */
 	A_GLOBL,
 	A_END,
 

cmd/w6c/txt.c

@@ -26,6 +26,7 @@ anames(int op)
 	case A_TEXT:	return "TEXT";
 	case A_DATA:	return "DATA";
 	case A_DATAW:	return "DATAW";
+	case A_DATAR:	return "DATAR";
 	case A_GLOBL:	return "GLOBL";
 	case A_END:	return "END";
 	case A_MOVQ:	return "MOVQ";