[AArch64] Make the TargetParser add CPU exts provided by the arch.

Otherwise, each CPU has to manually specify the extensions it supports,
even though they have to be a superset of the base arch extensions.
And when there's redundant data there's stale data, so most of the CPUs
lie about the features they support (almost none lists AEK_FP).

Instead, do the saner thing: add the optional extensions on top of the
base extensions provided by the architecture.

The ARM TargetParser has the same behavior.

Differential Revision: https://reviews.llvm.org/D32780

llvm-svn: 302078