AArch64: Add if condition in aarch64_function_value [PR96479]

Report diagnostic information instead of ICE if it generats fp/simd for
return register when fp/simd is disabled by -mgeneral-regs-only.

gcc/ChangeLog:

	* config/aarch64/aarch64.c (aarch64_function_value): Add if
	condition to check ag_mode after entering if condition of
	aarch64_vfp_is_call_or_return_candidate. If TARGET_FLOAT is
	set as false by -mgeneral-regs-only, report the diagnostic
	information of -mgeneral-regs-only imcompatible with the use
	of fp/simd register(s).

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/mgeneral-regs_1.c: Add the comment that
	-mgeneral-regs-only is compatible with the use of vector type
	used in the test case.