aarch64: return scalar fp8 values in fp registers

aarch64: return scalar fp8 values in fp registers

According to the aapcs64: If the argument is an 8-bit (...) precision
Floating-point or short vector type and the NSRN is less than 8, then the
argument is allocated to the least significant bits of register v[NSRN].

gcc/
	* config/aarch64/aarch64.cc
	(aarch64_vfp_is_call_or_return_candidate): use fp registers to
	return svmfloat8_t parameters.

gcc/testsuite/
	* gcc.target/aarch64/fp8_scalar_1.c: