selfhost: mirror writable .data + R_X86_64_64 across the wwstage

Bring the wwstage toolchain to parity with C-side DATAW / DATAR /
.data / .rela.data support. With this, w6c_ww + w6a_ww + w6l_ww can
compile, assemble and link `let g: str = "lit";` (and the scalar /
str / slice / struct globals that landed earlier) end-to-end, with
output that's byte-identical to the C-side pipeline.

w6a (types.ww / parse.ww / asm.ww / obj.ww):
  - A_DATAW + A_DATAR opcodes; parser learns `name+disp(SB)`;
    A_DATAR records an R_X86_64_64 reloc in .data via the new
    addrelocdata helper; areloc gains a `section` flag and asym
    an `isdata` flag; obj.ww splits relocs into .rela.text /
    .rela.data, emits .data PROGBITS + .rela.data conditionally,
    and shuffles section indices the same way cmd/w6a/obj.c does
    so byte output stays identical when no DATAW/DATAR are used.

w6l (sym.ww / obj.ww / pass.ww / out.ww / dynout.ww / main.ww):
  - lrel grows `section`; lsym grows `indata`; lobj tracks
    dataoff / datasize; lnk grows combined .data buffer;
  - obj.ww loads .data and .rela.data, registers data symbols
    with indata=1 and val shifted by the input's data_off, and
    the archive scanner includes both .text and .data globals;
  - pass.ww adds R_X86_64_64 (patch 8 bytes in .text or .data
    with sym_va + addend); relocate's signature becomes
    (textva, datava);
  - out.ww emits a second PT_LOAD (R+W) when datalen > 0, with
    .data at the page-aligned offset after .text;
  - dynout.ww refuses .data + -l/-L cleanly (matches the C-side
    error message);
  - main.ww computes text_va / data_va and passes both to
    relocate.

wcc cgen (cgen.ww / cgendecl.ww):
  - letpreintern walks top-level str-lets and interns the strlit
    BEFORE emitdatasection emits its DATA row, so emitletdataw
    can later look up the same label;
  - emitletdataw's 16B branch detects non-empty strlit init and
    emits the 8-zero + 8-LE-len DATAW plus a DATAR slot+0,strlit
    reloc, mirroring cmd/w6c/cgen.c.

Verified: `wwdump_ww -c` byte-matches `w6c` on a `let g: str =
"hello world\n";` fixture; `w6a_ww` and `w6l_ww` produce a
binary byte-identical to the C-side pipeline that runs and
prints "hello world". Bootstrap fixed-point holds (ww2 == ww3 ==
ww4), 26/26 tests green.

selfhost/cmd/w6l/sym.ww

@@ -7,8 +7,10 @@ use mem;
 
 type lsym = struct {
 	name:        str,
-	val:         u64,	// offset within combined .text once linked
+	val:         u64,	// offset within combined .text (or .data when
+				// indata=1) once linked
 	defined:     i32,	// 1 if some lobj defines this symbol
+	indata:      i32,	// 1 if defined in .data (writable globals)
 	owner:       *lobj,
 	idxinowner:  i32,
 	// Dynamic-linking fields. Set by resolve when an undefined sym
@@ -23,7 +25,8 @@ type lsym = struct {
 };
 
 type lrel = struct {
-	off:    u64,	// offset within combined .text
+	off:    u64,	// offset within the relocation's section
+	section: i32,	// 0 = .text, 1 = .data
 	kind:   i32,	// R_X86_64_*
 	sym:    *lsym,
 	addend: i64,
@@ -36,6 +39,8 @@ type lobj = struct {
 	len:      u64,
 	textoff:  u64,	// offset of .text in combined output
 	textsize: u64,
+	dataoff:  u64,	// offset of .data in combined output
+	datasize: u64,	// bytes contributed to combined .data (0 if none)
 	onext:    *lobj,
 };
 
@@ -64,6 +69,11 @@ type lnk = struct {
 	text:    *u8,	// combined .text
 	textcap: u64,
 	textlen: u64,
+	// Combined .data (writable). Empty unless any input .o has a
+	// .data PROGBITS section.
+	data:    *u8,
+	datacap: u64,
+	datalen: u64,
 	errs:    i32,
 	dynn:    i32,	// number of syms routed through PLT
 };