libstdc++: Implement std::byteswap for C++23

This patch attempts to implement P1272R4 (except for the std::bit_cast
changes in there which seem quite unrelated to this and will need to be
fixed on the compiler side).
While at least for GCC __builtin_bswap{16,32,64,128} should work fine
in constant expressions, I wonder about other compilers, so I'm using
a fallback implementation for constexpr evaluation always.
If you think that is unnecessary, I can drop the
__cpp_if_consteval >= 202106L &&
if !consteval
  {
and
  }
and reformat.
The fallback implementation is an attempt to make it work even for integral
types that don't have number of bytes divisible by 2 or when __CHAR_BIT__
is e.g. 16.

2021-11-28  Jakub Jelinek  <jakub@redhat.com>

	* include/std/bit (__cpp_lib_byteswap, byteswap): Define.
	* include/std/version (__cpp_lib_byteswap): Define.
	* testsuite/26_numerics/bit/bit.byteswap/byteswap.cc: New test.
	* testsuite/26_numerics/bit/bit.byteswap/version.cc: New test.