libstdc++: Factor out uses of __int128 into a type alias

libstdc++: Factor out uses of __int128 into a type alias

Since Ryu has the alias uint128_t for this same purpose, it seems best
for us to use this name as well, so as to minimize the amount of local
modifications we'd need to make to our copy of Ryu.  (In a subsequent
patch, we're going to remove Ryu's aliases so that it uses this one
defined in floating_to_chars.cc.)

libstdc++-v3/ChangeLog:

	* src/c++17/floating_to_chars.cc (uint128_t): New conditionally
	defined alias of unsigned __int128.
	(floating_type_traits_binary128::mantissa_t): Use uint128_t
	instead of unsigned __int128.
	(floating_type_traits<long double>::mantissa_t)
	[LONG_DOUBLE_KIND == LDK_IBM128]: Likewise.
	(get_ieee_repr): Likewise.  Make casts from uint_t to mantissa_t
	and uint32_t explicit.  Simplify the extraction of mantissa,
	exponent and sign bit.