Disable float128 tests on VxWorks, PR target/104253.

Disable float128 tests on VxWorks, PR target/104253.

In PR target/104253, it was pointed out the that test case added as part
of fixing the PR does not work on VxWorks because float128 is not
supported on that system.  I have modified the three tests for float128 so
that they are manually excluded on VxWorks systems.  In looking at the
code, I also added checks in check_effective_target_ppc_ieee128_ok to
disable the systems that will never support VSX instructions which are
required for float128 support (eabi, eabispe, darwin).

2022-04-07   Michael Meissner  <meissner@linux.ibm.com>

gcc/testsuite/
	PR target/104253
	* lib/target-supports.exp (check_ppc_float128_sw_available): Do
	not run float128 tests on VxWorks.
	(check_ppc_float128_hw_available): Likewise.
	(check_effective_target_ppc_ieee128_ok): Likewise.