[llvm][AArch64] apple-m4 is armv9.2-a (#98267)
But since SVE and friends have been added to the default extensions
list, and every CPU was opted into those extensions by default, we
couldn't correctly announce its architecutral version to the backend.
Additionally, we FEAT_MEC from llvm's "required" list for v9.0 to the
optional list for v9.2, as the spec considers it optional, and M4 does
not implement it. Similarly, fixes up several bugs w.r.t. FEAT_RME.
As a drive-by, I noticed that saphira did not have an
AArch64CPUTestParams entry, and thus added one.
