[AArch64] Add ARMv8.2-A Statistical Profiling Extension

The Statistical Profiling Extension is an optional extension to
ARMv8.2-A. Since it is an optional extension, I have added the
FeatureSPE subtarget feature to control it. The assembler-visible parts
of this extension are the new "psb csync" instruction, which is
equivalent to "hint #17", and a number of system registers.

Differential Revision: http://reviews.llvm.org/D15021

llvm-svn: 254401