IBM Z: Fix "+fvm" constraint with long doubles

IBM Z: Fix "+fvm" constraint with long doubles

When a long double is passed to an asm statement with a "+fvm"
constraint, a LRA loop occurs.  This happens, because LRA chooses the
widest register class in this case (VEC_REGS), but the code generated
by s390_md_asm_adjust() always wants FP_REGS.  Mismatching register
classes cause infinite reloading.

Fix by treating "fv" constraints as "v" in s390_md_asm_adjust().

gcc/ChangeLog:

	* config/s390/s390.c (f_constraint_p): Treat "fv" constraints
	as "v".

gcc/testsuite/ChangeLog:

	* gcc.target/s390/vector/long-double-asm-fprvrmem.c: New test.