[Flang] Split PowerPC-specific code out of IntrinsicCall into PPCIntrinsicCall

This patch moves PPC intrinsic generator code to PPCIntrinsicCall.cpp. In order to move PowerPC intrinsic code out of IntrinsicCall.cpp, we need to also move some declarations to IntrinsicCall.h. handlers[] and mathOperations[] were also chosen to be moved to the IntrinsicCall header. Similarly, ppcHandlers[] and ppcMathOperations[] were moved to the PPCIntrinsicCall header. There are future patches coming up that will introduce many new PPC intrinsics, these will now be defined in PPCIntrinsicCall.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D152460