[PowerPC][Future] Add missing changes for PC Realtive addressing

1. Use Subtarget.isUsingPCRelativeCalls() in LowerConstantPool to
check if using PCRelative addressing.

2. Change MO_GOT_FLAG = 32 to MO_GOT_FLAG = 8 in PPC.h to use
consecutive bits.

Differential Revision: https://reviews.llvm.org/D78406