IBM Z: Fix usage of "f" constraint with long doubles

IBM Z: Fix usage of "f" constraint with long doubles

After switching the s390 backend to store long doubles in vector
registers, "f" constraint broke when used with the former: long doubles
correspond to TFmode, which in combination with "f" corresponds to
hard regs %v0-%v15, however, asm users expect a %f0-%f15 pair.

Fix by using TARGET_MD_ASM_ADJUST hook to convert TFmode values to
FPRX2mode and back.

gcc/ChangeLog:

2020-12-14  Ilya Leoshkevich  <iii@linux.ibm.com>

	* config/s390/s390.c (f_constraint_p): New function.
	(s390_md_asm_adjust): Implement TARGET_MD_ASM_ADJUST.
	(TARGET_MD_ASM_ADJUST): Likewise.

gcc/testsuite/ChangeLog:

2020-12-14  Ilya Leoshkevich  <iii@linux.ibm.com>

	* gcc.target/s390/vector/long-double-asm-commutative.c: New
	test.
	* gcc.target/s390/vector/long-double-asm-earlyclobber.c: New
	test.
	* gcc.target/s390/vector/long-double-asm-in-out.c: New test.
	* gcc.target/s390/vector/long-double-asm-inout.c: New test.
	* gcc.target/s390/vector/long-double-asm-matching.c: New test.
	* gcc.target/s390/vector/long-double-asm-regmem.c: New test.
	* gcc.target/s390/vector/long-double-volatile-from-i64.c: New
	test.