[AArch64] Armv9-A implies FP16

As Armv9-a implies SVE2 it implies SVE (added in D141411) and so it
should also imply FP16, which this patch adds. This helps get the target
features correct when using `target("arch=armv9-a")` attributes.

There is also an adjustment to AssertSameExtensionFlags in this patch to
make it print cpu names, useful when the TargetParser unit tests are run
through lit to distinguish which cpu is failing.

Differential Revision: https://reviews.llvm.org/D142087