PowerPC: Require IBM long double for conversion test.

PowerPC: Require IBM long double for conversion test.

The test c-c++-common/dfp/convert-bfp-11.c explicit expects long double to use
the IBM 128-bit extended double format.  In particular, some of the tests
expect an infinity to be created if decimal values that are converted that are
too large for IBM extended double.  However, the numbers do fit in the range
for IEEE 128-bit format, since it has a larger exponent than the IBM 128-bit
format.  The test fails because an infinity is not generated.

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

	* c-c++-common/dfp/convert-bfp-11.c: Require IBM 128-bit long
	double.