[PowerPC] Export 16 byte load-store instructions

Export `lq`, `stq`, `lqarx` and `stqcx.` in preparation for implementing 16-byte lock free atomic operations on AIX.
Add a new register class `g8prc` for these instructions, since these instructions require even-odd register pair.

Reviewed By: nemanjai, jsji, #powerpc

Differential Revision: https://reviews.llvm.org/D103010