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/alloc.s

@@ -0,0 +1,24 @@
+// rt/alloc.s — page allocator via the mmap syscall.
+//
+// rt_alloc(n: u64) returns a *void aligned at a page boundary, sized
+// to the next page multiple. Pair with rt_free(p, n).
+//
+// We pin to PROT_READ|PROT_WRITE and MAP_PRIVATE|MAP_ANONYMOUS so
+// callers never have to plumb file descriptors through.
+
+TEXT rt_alloc,$0
+	MOVQ	DI, SI		// arg 1: length = caller's n
+	MOVQ	$0, DI		// arg 0: addr = NULL (kernel chooses)
+	MOVQ	$3, DX		// arg 2: prot = R|W
+	MOVQ	$34, R10	// arg 3: flags = MAP_PRIVATE|MAP_ANON
+	MOVQ	$-1, R8		// arg 4: fd = -1
+	MOVQ	$0, R9		// arg 5: offset = 0
+	MOVQ	$9, AX		// syscall: mmap
+	SYSCALL
+	RET
+
+TEXT rt_free,$0
+	// DI already holds ptr, SI already holds length
+	MOVQ	$11, AX		// syscall: munmap
+	SYSCALL
+	RET