aarch64: Update attributes of arm_fp16.h intrinsics

aarch64: Update attributes of arm_fp16.h intrinsics

Update the attributes of all intrinsics defined in arm_fp16.h to be
consistent with the attributes of the intrinsics defined in
arm_neon.h. Specifically, this means updating the attributes from:
  __extension__ static __inline <type>
  __attribute__ ((__always_inline__))
to:
  __extension__ extern __inline <type>
  __attribute__ ((__always_inline__, __gnu_inline__, __artificial__))

gcc/ChangeLog:

2021-03-18  Jonathan Wright  <jonathan.wright@arm.com>

	* config/aarch64/arm_fp16.h (__attribute__): Make intrinsic
	attributes consistent with those defined in arm_neon.h.