[AArch64] Move default extensions from clang Driver to TargetParser

The default extensions would be better added in the TargetParser, not by
the driver. This removes the addition of +i8mm and +bf16 features in the
driver as they are already added in 8.6/9.1 architectures. AEK_MOPS and
AEK_HBC have been added to 8.8/9.3 architectures to replace the need for
+hbc and +mops features.

Differential Revision: https://reviews.llvm.org/D141518