AArch64: Support for __builtin_arm_rbit() and __builtin_arm_rbit64().

__builtin_arm_rbit() and __builtin_arm_rbit64().

rdar://9283021

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211060 91177308-0d34-0410-b5e6-96231b3b80d8