IBM Z: Handle hard registers in s390_md_asm_adjust()

gen_fprx2_to_tf() and gen_tf_to_fprx2() cannot handle hard registers,
since the subregs they create do not pass validation.  Change
s390_md_asm_adjust() to manually copy between hard VRs and FPRs instead
of using these two functions.

gcc/ChangeLog:

	PR target/100217
	* config/s390/s390.c (s390_hard_fp_reg_p): New function.
	(s390_md_asm_adjust): Handle hard registers.

gcc/testsuite/ChangeLog:

	PR target/100217
	* gcc.target/s390/vector/long-double-asm-in-out-hard-fp-reg.c: New test.
	* gcc.target/s390/vector/long-double-asm-inout-hard-fp-reg.c: New test.