[ARM] Add PostRAScheduler option

This patch adds the option to allow also using the PostRA scheduler,
which brings the ARM backend inline with AArch64 targets. The
SchedModel can also set 'PostRAScheduler', as the R52 does, so also
query this property in the overridden function.

Differential Revision: https://reviews.llvm.org/D36866

llvm-svn: 311162