ARM: intrinsic support for rbit.

We already have an ARMISD node. Create an intrinsic to map to it so we can
add support for the frontend __rbit() intrinsic.

rdar://9283021
AArch64: Add backend intrinsic for rbit.

Define an intrinsic for the frontend to use and pattern match it to
the RBIT instruction.

rdar://9283021
ARM: Support for __builtin_arm_rbit() intrinsic.

Reverse the bits in a word. Maps to the RBIT instruction.

rdar://9283021
AArch64: Support for __builtin_arm_rbit() and __builtin_arm_rbit64().

__builtin_arm_rbit() and __builtin_arm_rbit64().

rdar://9283021