[PEI] Switch to backwards frame index elimination by default

Also rename the flag from supportsBackwardScavenger to
eliminateFrameIndicesBackwards to reflect what it actually does.

X86 is the only target still using forwards frame index elimination.
This will not block removing support for forwards register scavenging,
because X86 does not use the register scavenger.

Differential Revision: https://reviews.llvm.org/D156983