[ARM] Add mrrc/mrrc2 co-processor intrinsics

MRRC/MRRC2 instruction writes to two registers. The
intrinsic definition returns a single uint64_t to
represent the write, this is a compact way of
representing a write to two 32 bit registers,
the alternative might have been two return a
struct of 2 uint32_t's but this isn't as nice.

Differential Revision: 

llvm-svn: 272544