ww: import toolchain — C bootstrap + ww-side self-host (phases 0-10)

C bootstrap (phases 0-9):
  cmd/wwc, cmd/6c, cmd/6a, cmd/6l, cmd/ww, rt, lib/*.

ww-side self-host (phase 10):
  selfhost/cmd/wwc — ww-cgen frontend; bootstrap fixed point.
  selfhost/cmd/6a  — assembler; byte-identical to C 6a (test 991).
  selfhost/cmd/6l  — linker w/ archive (.a) support; byte-identical
                     to C 6l (test 992).
  selfhost/cmd/ww  — driver (build/run/version); byte-identical to
                     C ww (test 993).

make test: 15/15. make bootstrap: ww2.s == ww3.s, ww2.o == ww3.o,
ww2 == ww3 byte-identical, with the full ww-tooled chain.

rt/streq.s

@@ -0,0 +1,27 @@
+// rt/streq.s — string equality at the runtime layer.
+//
+// Called from cgen for `a == b` / `a != b` when both operands are str.
+// SysV ABI: a.ptr in DI, a.len in SI, b.ptr in DX, b.len in CX.
+// Returns 1 in AX if equal, 0 otherwise. Caller flips for !=.
+
+TEXT rt_streq,$0
+	CMPQ	CX, SI			// lengths must match
+	JNE	rt_streq_no
+	// SI now holds the (shared) length; iterate while SI > 0
+rt_streq_loop:
+	CMPQ	$0, SI
+	JE	rt_streq_yes
+	MOVZBQ	(DI), AX
+	MOVZBQ	(DX), R8
+	CMPQ	R8, AX
+	JNE	rt_streq_no
+	ADDQ	$1, DI
+	ADDQ	$1, DX
+	SUBQ	$1, SI
+	JMP	rt_streq_loop
+rt_streq_yes:
+	MOVQ	$1, AX
+	RET
+rt_streq_no:
+	MOVQ	$0, AX
+	RET