IBM Z: Introduce __LONG_DOUBLE_VX__ macro

IBM Z: Introduce __LONG_DOUBLE_VX__ macro

Give end users the opportunity to find out whether long doubles are
stored in floating-point register pairs or in vector registers, so that
they could fine-tune their asm statements.

gcc/ChangeLog:

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

	* config/s390/s390-c.c (s390_def_or_undef_macro): Accept
	callables instead of mask values.
	(struct target_flag_set_p): New predicate.
	(s390_cpu_cpp_builtins_internal): Define or undefine
	__LONG_DOUBLE_VX__ macro.

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

gcc/testsuite/ChangeLog:

	* gcc.target/s390/vector/long-double-vx-macro-off-on.c: New test.
	* gcc.target/s390/vector/long-double-vx-macro-on-off.c: New test.