ARM: Implement __builtin_arm_nop intrinsic

This patch implements __builtin_arm_nop intrinsic for AArch32 and AArch64,
which generates hint 0x0, the alias of NOP instruction.

This intrinsic is necessary to implement ACLE __nop intrinsic.

Differential Revision: http://reviews.llvm.org/D4495

llvm-svn: 212947