[AArch64] Enable non-leaf frame pointer elimination.

Summary:
This change enables frame pointer elimination in non-leaf functions.
The -fomit-frame-pointer option still needs to be used when compiling
via clang (or an equivalent method of not setting the
'no-frame-pointer-elim*' function attributes if generating llvm IR via
some other method) to take advantage of this optimization.

This change should be NFC when compiling via clang without
-fomit-frame-pointer.

Reviewers: t.p.northover

Subscribers: aemerson, rengolin, tberghammer, qcolombet, llvm-commits, danalbert, mcrosier, srhines

Differential Revision: http://reviews.llvm.org/D17730

llvm-svn: 262495