boost::math::cstdfloat::detail::float_internal128_t

Hello,
in boost::math::cstdfloat there are several inconsistencies with __float128/_Quad. Generally these are only available with gcc and intel. Since I don't have an intel compiler myself, you might have to modify this path; for now it should behave like the previous implementations.

types
intel https://community.intel.com/t5/Intel-C-Compiler/C-Compiler-128-bit-floating-point/m-p/1216949
- WINDOWS: float_internal128_t = _quad
- OTHER: float_internal128_t = __float128

defines/constants
gcc
- the constants from or builtins can be used
- signaling NaN is supported

functions
modified:
- gcc: nanq does not support payload https://github.com/gcc-mirror/gcc/blob/master/libquadmath/math/nanq.c
--> #define BOOST_CSTDFLOAT_FLOAT128_NAN __builtin_nanq
- hypot(x, y, z) correct result even with large values; slower than previous naive implementation
- isfinite(x) branch-free

limits
All functions can now be provided constexpr noexcept.

future
- branch-free isgreater, isgreaterequal, ... I don't understand the BOOST_PREVENT_MACRO_SUBSTITUTION inside the functions, otherwise I could have adjusted that right away.
- With Matt's boost::charconv the IO functions can be provided more efficiently (operator<<, operator>>, to_string, ...).
- It might be useful to provide quadmath as a separate sublibrary in boost::math, e.g. to avoid the #define/#undef orgies in .

[cstdfloat_types.hpp.txt](https://github.com/boostorg/math/files/11805871/cstdfloat_types.hpp.txt)
[cstdfloat_cmath.hpp.txt](https://github.com/boostorg/math/files/11805874/cstdfloat_cmath.hpp.txt)
[cstdfloat_limits.hpp.txt](https://github.com/boostorg/math/files/11805875/cstdfloat_limits.hpp.txt)

thx
Gero