testsuite - Some float64 and float32x test require double64plus.

Some of the float64 and float32x test cases are using double built-ins
and hence require double64plus resp. that double is at least as good
as float32x (double_float32xplus).

gcc/testsuite/
	* lib/target-supports.exp (check_effective_target_double_float32xplus):
	New proc.
	* gcc.dg/torture/float32x-builtin.c: Add
	dg-require-effective-target double_float32xplus.
	* gcc.dg/torture/float32x-tg-2.c: Same.
	* gcc.dg/torture/float32x-tg.c: Same.
	* gcc.dg/torture/float64-builtin.c: Add
	dg-require-effective-target double64plus.
	* gcc.dg/torture/float64-tg-2.c: Same.
	* gcc.dg/torture/float64-tg.c: Same.