runtime/internal/atomic: crash on unaligned 64-bit ops on 32-bit MIPS

This check was originally implemented by Vladimir in
https://go-review.googlesource.com/c/31489/1/src/runtime/internal/atomic/atomic_mipsx.go#30
but removed due to my comment (Sorry!). This CL adds it back.

Fixes #17786.

Change-Id: I7ff4c2539fc9e2afd8199964b587a8ccf093b896
Reviewed-on: https://go-review.googlesource.com/33431
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/internal/atomic/atomic_mipsx.go

@@ -25,7 +25,11 @@ func spinUnlock(state *uint32)
 
 //go:nosplit
 func lockAndCheck(addr *uint64) {
-	//  force dereference before taking lock
+	// ensure 8-byte alignement
+	if uintptr(unsafe.Pointer(addr))&7 != 0 {
+		addr = nil
+	}
+	// force dereference before taking lock
 	_ = *addr
 
 	spinLock(&lock.state)

src/runtime/internal/atomic/atomic_test.go

@@ -87,8 +87,8 @@ func TestUnaligned64(t *testing.T) {
 		if unsafe.Sizeof(int(0)) != 4 {
 			t.Skip("test only runs on 32-bit systems")
 		}
-	case "amd64p32", "mips", "mipsle":
-		// amd64p32 and mips can handle unaligned atomics.
+	case "amd64p32":
+		// amd64p32 can handle unaligned atomics.
 		t.Skipf("test not needed on %v", runtime.GOARCH)
 	}
 

src/sync/atomic/atomic_test.go

@@ -1401,8 +1401,8 @@ func TestUnaligned64(t *testing.T) {
 		if unsafe.Sizeof(int(0)) != 4 {
 			t.Skip("test only runs on 32-bit systems")
 		}
-	case "amd64p32", "mips", "mipsle":
-		// amd64p32 and mips can handle unaligned atomics.
+	case "amd64p32":
+		// amd64p32 can handle unaligned atomics.
 		t.Skipf("test not needed on %v", runtime.GOARCH)
 	}