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/
2020-12-03  Michael Meissner  <meissner@linux.ibm.com>

	* lib/target-supports.exp
	(check_effective_target_ppc_long_double_ibm): New function.
	(check_effective_target_ppc_long_double_ieee): New function.
	(check_effective_target_ppc_long_double_128bit): New function.
	(check_effective_target_ppc_long_double_64bit): New function.