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/start.s

@@ -0,0 +1,16 @@
+// rt/start.s — process entry. Linux sets up the stack so that on
+// entry [SP] holds argc, [SP+8] starts argv (NULL-terminated array
+// of *u8), [SP+8+(argc+1)*8] starts envp.
+//
+// We pull argc into DI and the argv pointer into SI, then jump to
+// `main`. ww programs that declare `fn main(argc: i32, argv: **u8)
+// i32` see them; programs declaring `fn main() i32` simply ignore
+// the regs. Either way, main's return value is fed to the exit
+// syscall.
+TEXT _start,$0
+	MOVQ	(SP), DI
+	LEAQ	8(SP), SI
+	CALL	main(SB)
+	MOVQ	AX, DI
+	MOVQ	$60, AX
+	SYSCALL