benchtests: Run _Float128 tests only on architectures that support it

__float128 is a non-standard name and is not available on some architectures
(like aarch64 or s390x) even though they may support the standard _Float128
type.  Other architectures (like armv7) don't support quad-precision
floating-point operations at all.

This commit replaces benchtests references to __float128 with _Float128 and
runs the corresponding tests only on architectures that support it.