libstdc++: Remove _GLIBCXX_USE_INT128 autoconf macro

We don't need to decide whether to use __int128 when running configure,
we can do so at compilation time by seeing if __SIZEOF_INT128__ is
defined and if it's greater than __SIZEOF_LONG_LONG__.

This removes another unnecessary architecture-specific config macro in
<bits/c++config.h>, so the same header can work for 32-bit or 64-bit
compilation on AIX.

libstdc++-v3/ChangeLog:

	* acinclude.m4 (GLIBCXX_ENABLE_INT128_FLOAT128): Remove
	checks for __int128 and rename to GLIBCXX_ENABLE_FLOAT128.
	* config.h.in: Regenerate.
	* configure: Regenerate.
	* configure.ac: Adjust to use GLIBCXX_ENABLE_FLOAT128.
	* include/bits/random.h (_Select_uint_least_t<s, 1>):
	Use __SIZEOF_INT128__ to decide whether to use __int128.
	* include/std/charconv (__to_chars_unsigned_type): Likewise.