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/abort.s

@@ -0,0 +1,17 @@
+// rt/abort.s — write a string to stderr and exit(1).
+//
+// Args (SysV str-by-value): DI = msg.ptr, SI = msg.len.
+// We never return.
+
+TEXT rt_abort,$0
+	// reorder args for the write syscall: rdi=fd, rsi=ptr, rdx=len.
+	MOVQ	SI, DX		// DX = len
+	MOVQ	DI, SI		// SI = ptr
+	MOVQ	$2, DI		// DI = stderr
+	MOVQ	$1, AX		// AX = sys_write
+	SYSCALL
+	// exit(1)
+	MOVQ	$1, DI
+	MOVQ	$60, AX		// sys_exit
+	SYSCALL
+	RET