s390x: Fix s390_md_asm_adjust handling of long doubles [PR112753]

Commit 466b100e introduced a bug in
s390_md_asm_adjust if vector extensions are not available.  Fix the
control flow of this function to not adjust long double values.

gcc/ChangeLog:

	PR target/112753
	* config/s390/s390.cc (s390_md_asm_adjust): Return after dealing
	with the outputs, if no further processing of long doubles is
	required.

gcc/testsuite/ChangeLog:

	* gcc.target/s390/pr112753.c: New test.