Fix iseqsig for _FloatN and _FloatNx in C++ with GCC 13

Fix iseqsig for _FloatN and _FloatNx in C++ with GCC 13

With GCC 13, _FloatN and _FloatNx types, when they exist, are distinct
types like they are in C with GCC 7 and later, rather than typedefs
for types such as float, double or long double.

This breaks the templated iseqsig implementation for C++ in <math.h>,
when used with types that were previously implemented as aliases.  Add
the necessary definitions for _Float32, _Float64, _Float128 (when the
same format as long double), _Float32x and _Float64x in this case, so
that iseqsig can be used with such types in C++ with GCC 13 as it
could with previous GCC versions.

Also add tests for calling iseqsig in C++ with arguments of such types
(more minimal than existing tests, so that they can work with older
GCC versions and without relying on any C++ library support for the
types or on hardcoding details of their formats).  The LDBL_MANT_DIG
!= 106 conditionals on some tests are because the type-generic
comparison macros have undefined behavior when neither argument has a
type whose set of values is a subset of those for the type of the
other argument, which applies when one argument is IBM long double and
the other is an IEEE format wider than binary64.

Tested with build-many-glibcs.py glibcs build for aarch64-linux-gnu
i686-linux-gnu mips-linux-gnu mips64-linux-gnu-n32 powerpc-linux-gnu
powerpc64le-linux-gnu x86_64-linux-gnu.