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/pass.ww

@@ -14,6 +14,7 @@ use os;
 use sym;
 use dyn;
 
+def R_X86_64_64:    i32 = 1;
 def R_X86_64_PC32:  i32 = 2;
 def R_X86_64_PLT32: i32 = 4;
 
@@ -80,7 +81,15 @@ fn patchu32(p: *u8, v: u32) void = {
 	p[3] = ((v >> 24u32) & 255u32): u8;
 };
 
-export fn relocate(l: *lnk, base: u64) i32 = {
+fn patchu64(p: *u8, v: u64) void = {
+	let i: i32 = 0;
+	for (i < 8) {
+		p[i] = ((v >> (i: u64 * 8u64)) & 255u64): u8;
+		i += 1;
+	};
+};
+
+export fn relocate(l: *lnk, textva: u64, datava: u64) i32 = {
 	let r: *lrel = l.rels;
 	for (r != nil) {
 		if (r.sym != nil) {
@@ -91,21 +100,30 @@ export fn relocate(l: *lnk, base: u64) i32 = {
 				continue;
 			};
 			if (r.sym.defined != 0) {
+				let symva: u64 = textva + r.sym.val;
+				if (r.sym.indata != 0) { symva = datava + r.sym.val; };
 				let k: i32 = r.kind;
 				if (k == R_X86_64_PC32) {
-					let site: u64 = base + r.off;
-					let target: i64 = (base + r.sym.val): i64;
-					let rel: i64 = (target - site: i64) + r.addend;
+					let site: u64 = textva + r.off;
+					let rel: i64 = (symva: i64 - site: i64) + r.addend;
 					patchu32(l.text + r.off, rel: u32);
 				} else { if (k == R_X86_64_PLT32) {
-					let site: u64 = base + r.off;
-					let target: i64 = (base + r.sym.val): i64;
-					let rel: i64 = (target - site: i64) + r.addend;
+					let site: u64 = textva + r.off;
+					let rel: i64 = (symva: i64 - site: i64) + r.addend;
 					patchu32(l.text + r.off, rel: u32);
+				} else { if (k == R_X86_64_64) {
+					// Absolute 64-bit. Currently used only
+					// for DATAR slots in .data.
+					let v: u64 = (symva: i64 + r.addend): u64;
+					if (r.section == 1) {
+						patchu64(l.data + r.off, v);
+					} else {
+						patchu64(l.text + r.off, v);
+					};
 				} else {
 					os.write(2, "w6l: unsupported reloc kind\n".ptr, 27u64);
 					l.errs += 1;
-				};};
+				};};};
 			};
 		};
 		r = r.rnext;