[CGP] Add generic TargetLowering::shouldAlignPointerArgs() implementation

This function was added for ARM targets, but aligning global/stack pointer
arguments passed to memcpy/memmove/memset can improve code size and
performance for all targets that don't have fast unaligned accesses.
This adds a generic implementation that adjusts the alignment to pointer
size if unaligned accesses are slow.
Review D134168 suggests that this significantly improves performance on
synthetic benchmarks such as Dhrystone on RV32 as it avoids memcpy() calls.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D134282