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/syscall.s

@@ -0,0 +1,16 @@
+// rt/syscall.s — Linux amd64 syscall trampoline. Args go in
+// DI, SI, DX, R10, R8, R9 (the kernel ABI; userspace 4th is CX). We
+// keep things minimal: rt_syscall(num, a1, a2, a3, a4, a5) → result.
+//
+// The first ww arg lands in DI (caller side); to use it as the
+// syscall number we move it into AX. The 4th C-ABI arg comes in CX
+// and must be moved to R10 for the kernel.
+TEXT rt_syscall,$0
+	MOVQ	DI, AX
+	MOVQ	SI, DI
+	MOVQ	DX, SI
+	MOVQ	CX, DX
+	MOVQ	R8, R10
+	MOVQ	R9, R8
+	SYSCALL
+	RET