[AArch64] Specify fp16 support for Cortex-A55 and Cortex-A75

[AArch64] Specify fp16 support for Cortex-A55 and Cortex-A75

The Cortex-A55 and Cortex-A75 processors support the fp16 extension.
We already specify them as such in the arm port.
This patch makes aarch64 consistent on this front.

Bootstrapped and tested on aarch64-none-linux-gnu.
Manually checked that compiling with aarch64-none-linux-gnu-gcc -mcpu=cortex-a55 -dM -E - < /dev/null
shows __ARM_FEATURE_FP16_VECTOR_ARITHMETIC and __ARM_FEATURE_FP16_SCALAR_ARITHMETIC being specified
as expected whereas they were not before this patch.

	* config/aarch64/aarch64-cores.def (cortex-a55, cortex-a75,
	cortex-a75.cortex-a55): Specify AARCH64_FL_F16 in the arch features.

From-SVN: r255624