[AArch64] Correct features for Arm Cortex-A78C, Cortex-X1C and Cortex-X2 (#76932)

Remove AArch64::AEK_FP16ML from Arm Cortex-A78C definition, as
this is not supported, according to the Technical Reference Manual:
   https://developer.arm.com/documentation/102226/latest/

Also add AArch64::AEK_FLAGM (Flag Manipulation) to Arm Cortex-X1C
and Arm Cortex-X2 as these were missing previously, but are
supported, according to the Technical Reference Manuals:
   https://developer.arm.com/documentation/101968/latest/
   https://developer.arm.com/documentation/101803/latest/

Fixes #62383