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.

cmd/6c/swt.c

@@ -0,0 +1,8 @@
+/*
+ * swt.c — switch-statement lowering. Stub for now: cgen falls
+ * through to a no-op for N_SWITCH. When we add a real lowering, it
+ * will live here, mirroring Plan 9 6c's pswt.c.
+ */
+#include "gc.h"
+
+void	swt_lower(Cg *c, Node *n) { (void)c; (void)n; }