ARM32: Default to {ldrexd,strexd} in StubRoutines::atomic_{load|store}_long

Current ARM32 is one of few remaining uses of `os::is_MP`, the rest is removed by JDK-8188764. There are some interesting bugs in OS/libc that might give incorrect `os::is_MP` sometimes, e.g. in containers. Instead of risking it, we can default to {ldrexd,strexd} for ARMv7 (which always have them), and leave the `os::is_MP` path for ARMv6 (for which this is the only remaining way to load the 64-bit long).