[AArch64] Change __ARM_FEATURE_FP16FML macro name to __ARM_FEATURE_FP16_FML

The "Arm C Language extensions" document (the current version can be
found at https://developer.arm.com/documentation/101028/0012/?lang=en)
states that the name of the feature test macro for the FP16 FML extension
is __ARM_FEATURE_FP16_FML.

Differential Revision: https://reviews.llvm.org/D101532