stack overflow handler specific openbsd fix.

On this platform, when doing stack allocation, MAP_STACK is needed
 otherwise the mapping fails.

library/std/src/sys/unix/stack_overflow.rs

@@ -143,14 +143,15 @@ mod imp {
     }
 
     unsafe fn get_stackp() -> *mut libc::c_void {
-        let stackp = mmap(
-            ptr::null_mut(),
-            SIGSTKSZ + page_size(),
-            PROT_READ | PROT_WRITE,
-            MAP_PRIVATE | MAP_ANON,
-            -1,
-            0,
-        );
+        // OpenBSD requires this flag for stack mapping
+        // otherwise the said mapping will fail as a no-op on most systems
+        // and has a different meaning on FreeBSD
+        #[cfg(any(target_os = "openbsd", target_os = "netbsd", target_os = "linux",))]
+        let flags = MAP_PRIVATE | MAP_ANON | libc::MAP_STACK;
+        #[cfg(not(any(target_os = "openbsd", target_os = "netbsd", target_os = "linux",)))]
+        let flags = MAP_PRIVATE | MAP_ANON;
+        let stackp =
+            mmap(ptr::null_mut(), SIGSTKSZ + page_size(), PROT_READ | PROT_WRITE, flags, -1, 0);
         if stackp == MAP_FAILED {
             panic!("failed to allocate an alternative stack: {}", io::Error::last_os_error());
         }