PowerPC: Add long double target-supports.

PowerPC: Add long double target-supports.

This patch adds 3 target supports to test what type of PowerPC long double is
used by the test:

   1)	Long double uses the IBM 128-bit extended double format;
   2)	Long double uses the IEEE 128-bit format; (and)
   3)	Long double uses the 64-bit format.

gcc/testsuite/
2020-11-30  Michael Meissner  <meissner@linux.ibm.com>

	* lib/target-supports.exp (check_ppc_long_double_ibm): New
	function.
	(check_ppc_long_double_ieee): New function.
	(check_ppc_long_double_64bit): New function.
	(is-effective-target): Add ppc_long_double_ibm,
	ppc_long_double_ieee, and ppc_long_double_64bit.