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.

lib/encoding/utf8/utf8.ww

@@ -0,0 +1,14 @@
+// encoding/utf8 — UTF-8 helpers. RFC 3629; we only handle the legal
+// subset (no over-long encodings, no surrogates).
+
+def MAX:  rune = 1114111;	// 0x10FFFF
+def BAD:  rune = -1;
+
+export fn runelen(r: rune) i32 = {
+	if (r < 0)         { return -1; };
+	if (r < 128)       { return 1; };
+	if (r < 2048)      { return 2; };
+	if (r < 65536)     { return 3; };
+	if (r <= MAX)      { return 4; };
+	return -1;
+};