[AArch64] Enable PostRAScheduler for AArch64 generic build

This patch enables PostRAScheduler specifically for AArch64 generic build,
which is beneficial from the performance perspective.
Speedups up to 2 to 7% for some benchmarks on A57 and A53 are observed.
Also benchmarks from LLVM test-suite did not regress.

Differential Revision: http://reviews.llvm.org/D15557

llvm-svn: 255896