Force IBM long double for conversion test on PowerPC.

Force IBM long double for conversion test on PowerPC.

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.

This patch explicitly sets the long double type to IBM extended double.

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>

	* c-c++-common/dfp/convert-bfp-11.c: Force using IBM 128-bit long
	double.  Remove check for 64-bit long double.