runtime: use MAP_ANON in sigstack check

MAP_ANON is the deprecated but more portable spelling of
MAP_ANONYMOUS. Use MAP_ANON to un-break the Darwin 10.10 builder.

Updates #22930.

Change-Id: Iedd6232b94390b3b2a7423c45cdcb25c1a5b3323
Reviewed-on: https://go-review.googlesource.com/81615
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/testdata/testprogcgo/sigstack.go

@@ -25,7 +25,7 @@ extern void SigStackCallback();
 
 static void* WithSigStack(void* arg __attribute__((unused))) {
 	// Set up an alternate system stack.
-	void* base = mmap(0, SIGSTKSZ, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_STACK, -1, 0);
+	void* base = mmap(0, SIGSTKSZ, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANON|MAP_STACK, -1, 0);
 	if (base == MAP_FAILED) {
 		perror("mmap failed");
 		abort();