Fix tests that require IBM 128-bit long double

Fix tests that require IBM 128-bit long double

2021-09-03  Michael Meissner  <meissner@linux.ibm.com>

gcc/testsuite/
	PR target/94630
	* gcc.target/powerpc/pr70117.c: Specify that we need the long double
	type to be IBM 128-bit.  Remove the code to use __ibm128.
	Backport from master 2021-08-25.
	* c-c++-common/dfp/convert-bfp-11.c: Specify that we need the long
	double type to be IBM 128-bit.  Run the test at -O2 optimization.
	Backport from master 2021-08-25.
	* lib/target-supports.exp (add_options_for_long_double_ibm128): New
	function.  Backport from master 2021-08-25.
	(check_effective_target_long_double_ibm128): New function.
	(add_options_for_long_double_ieee128): New function.
	(check_effective_target_long_double_ieee128): New function.
	(add_options_for_long_double_64bit): New function.
	(check_effective_target_long_double_64bit): New function.