Add long double target-supports on PowerPC.

Add long double target-supports on PowerPC.

This patch add several more selections to target-supports.exp:

    *	3 selections if we can change the long double format to a particular
	value.
    *	3 functions to return the options needed to switch the long double
	format for use with dg-add-options.

I have run tests on a little endian power9 system with 3 compilers.  There
were no regressions with these patches, and the two tests in the following
patches now work if the default long double is not IBM 128-bit:

    *	One compiler used the default IBM 128-bit format;
    *	One compiler used the IEEE 128-bit format; (and)
    *	One compiler used 64-bit long doubles.

I have also tested compilers on a big endian power8 system with a compiler
defaulting to power8 code generation and another with the default cpu
set.  There were no regressions.

Can I check this patch into the master branch?

gcc/testsuite/
2021-04-28  Michael Meissner  <meissner@linux.ibm.com>

	* lib/target-supports.exp
	(add_options_for_ppc_long_double_override_ibm128): New function.
	(check_effective_target_ppc_long_double_override_ibm128): New
	function.
	(add_options_for_ppc_long_double_override_ieee128): New function.
	(check_effective_target_ppc_long_double_override_ieee128): New
	function.
	(add_options_for_ppc_long_double_override_64bit): New function.
	(check_effective_target_ppc_long_double_override_64bit): New
	function.