[PEI, AArch64] Use empty spaces in stack area for local stack slot allocation.

Summary:
If the target requests it, use emptry spaces in the fixed and
callee-save stack area to allocate local stack objects.

AArch64: Change last callee-save reg stack object alignment instead of
size to leave a gap to take advantage of above change.

Reviewers: t.p.northover, qcolombet, MatzeB

Subscribers: rengolin, mcrosier, llvm-commits, aemerson

Differential Revision: http://reviews.llvm.org/D20220

llvm-svn: 271527