[AArch64] Add support for the 'R' architecture profile.

This change introduces subtarget features to predicate certain
instructions and system registers that are available only on
'A' profile targets. Those features are not present when
targeting a generic CPU, which is the default processor.

In other words the generic CPU now means the intersection of
'A' and 'R' profiles. To maintain backwards compatibility we
enable the features that correspond to -march=armv8-a when the
architecture is not explicitly specified on the command line.

References: https://developer.arm.com/documentation/ddi0600/latest

Differential Revision: https://reviews.llvm.org/D110065