runtime: don't use MAP_STACK in SigStack test

On DragonFly mmap with MAP_STACK returns the top of the region, not
the bottom. Rather than try to cope, just don't use the flag anywhere.

Fixes #23061

Change-Id: Ib5df4dd7c934b3efecfc4bc87f8989b4c37555d7
Reviewed-on: https://go-review.googlesource.com/83035
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/testdata/testprogcgo/sigstack.go

@@ -17,15 +17,11 @@ package main
 #include <stdlib.h>
 #include <sys/mman.h>
 
-#ifndef MAP_STACK
-#define MAP_STACK 0
-#endif
-
 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_ANON|MAP_STACK, -1, 0);
+	void* base = mmap(0, SIGSTKSZ, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANON, -1, 0);
 	if (base == MAP_FAILED) {
 		perror("mmap failed");
 		abort();