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/path/path.ww

@@ -0,0 +1,15 @@
+// path — filesystem path manipulation. UTF-8 paths, '/' separator.
+
+export fn isabs(p: str) bool = {
+	if (p.len == 0) { return false; };
+	return p[0] == ('/': u8);
+};
+
+export fn lastindex(p: str, c: u8) i32 = {
+	let i: i32 = p.len - 1;
+	for (i >= 0) {
+		if (p[i] == c) { return i; };
+		i -= 1;
+	};
+	return -1;
+};