aarch64: Enable FP16 feature by default for Armv9

This patch adds the feature bit for FP16 to the feature set for Armv9 since
Armv9 requires SVE to be implemented and SVE requires FP16 to be implemented.

2022-03-22  Andre Vieira  <andre.simoesdiasvieira@arm.com>

	* config/aarch64/aarch64.h (AARCH64_FL_FOR_ARCH9): Add FP16 feature
	bit.