MIPS: Use compact branch for LL/SC loops on MIPSr6+

When targeting MIPSr6 or higher make use of a compact branch in LL/SC
loops, preventing the insertion of a delay slot nop that only serves to
waste space.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org
Cc: Huacai Chen <chenhc@lemote.com>
Cc: Jiaxun Yang <jiaxun.yang@flygoat.com>
Cc: linux-kernel@vger.kernel.org