Continuing the preparation for additional _FloatN / _FloatNx type
support, this patch improves how <tgmath.h> handles such types.
Use of #error is added for cases of distinct types that are not
supported by the header, to indicate that additional work on the
header would be needed if, for example, _Float16 support were added to
glibc. Given that #error, types with the same format as other types
are handled automatically by the sizeof-based logic, so the only case
needing special handling is that where _Float64x exists, has the same
format as _Float128, does not have the same format as long double, and
is not a typedef for _Float128. In this case (which will apply for
powerpc64le once _Float64x support is added to glibc), the
__builtin_types_compatible_p calls testing for _Float128 need
corresponding calls testing for _Float64x, which this patch adds.