openbsd: don't reallocate a guard page on the stack.

the kernel currently enforce that a stack is immutable. calling mmap(2) or 
mprotect(2) to change it will result in EPERM, which generate a panic!().

so just do like for Linux, and trust the kernel to do the right thing.

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

@@ -767,6 +767,16 @@ pub mod guard {
             const GUARD_PAGES: usize = 1;
             let guard = guardaddr..guardaddr + GUARD_PAGES * page_size;
             Some(guard)
+        } else if cfg!(target_os = "openbsd") {
+            // OpenBSD stack already includes a guard page, and stack is
+            // immutable.
+            //
+            // We'll just note where we expect rlimit to start
+            // faulting, so our handler can report "stack overflow", and
+            // trust that the kernel's own stack guard will work.
+            let stackptr = get_stack_start_aligned()?;
+            let stackaddr = stackptr.addr();
+            Some(stackaddr - page_size..stackaddr)
         } else {
             // Reallocate the last page of the stack.
             // This ensures SIGBUS will be raised on