runtime: fix TestPhysPageSize on AIX

AIX doesn't allow to mmap an address range which is already mmap.
    Therefore, once the region has been allocated, it must munmap before
    being able to play with it.
    The corresponding Go Toolchain patch is CL 174059.
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/174138

From-SVN: r270615

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-cb6fb7285bac72389bdce7ecfe87f9366022571a
+b117b468264665cfe6ec2cf3affb48330a704fa7
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/runtime/runtime_mmap_test.go

@@ -32,6 +32,11 @@ func TestPhysPageSize(t *testing.T) {
 		t.Fatalf("Mmap: %v", err)
 	}
 
+	if runtime.GOOS == "aix" {
+		// AIX does not allow mapping a range that is already mapped.
+		runtime.Munmap(unsafe.Pointer(uintptr(b)), 2*ps)
+	}
+
 	// Mmap should fail at a half page into the buffer.
 	_, err = runtime.Mmap(unsafe.Pointer(uintptr(b)+ps/2), ps, 0, runtime.MAP_ANON|runtime.MAP_PRIVATE|runtime.MAP_FIXED, -1, 0)
 	if err == 0 {