[LLVM - ARM/AArch64] Add ACLE special register intrinsics

[LLVM - ARM/AArch64] Add ACLE special register intrinsics

This patch implements LLVM support for the ACLE special register intrinsics in
section 10.1, __arm_{w,r}sr{,p,64}.

This patch is intended to lower the read/write_register instrinsics, used to
implement the special register intrinsics in the clang patch for special
register intrinsics (see http://reviews.llvm.org/D9697), to ARM specific
instructions MRC,MCR,MSR etc. to allow reading an writing of coprocessor
registers in AArch32 and AArch64. This is done by inspecting the register
string passed to the intrinsic and then lowering to the appropriate
instruction.

Patch by Luke Cheeseman.

Differential Revision: http://reviews.llvm.org/D9699

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237579 91177308-0d34-0410-b5e6-96231b3b80d8