libstdc++: Add a fallback 128-bit integer class type and use it

This implements a minimal integer class type that emulates 128-bit
unsigned arithmetic using a pair of 64-bit integers, which the
floating-point std::to_chars implementation then uses as a drop-in
replacement for unsigned __int128 on targets that lack the latter.
After this patch, we now fully support formatting of large long double
types on such targets.

Since Ryu performs 128-bit division/modulus only by 2, 5 and 10, this
integer class type supports only these divisors rather than general
division/modulus.

libstdc++-v3/ChangeLog:

	* src/c++17/floating_to_chars.cc: Simplify the file as if
	__SIZEOF_INT128__ is always defined.
	[!defined __SIZEOF_INT128__]: Include "uint128_t.h".  Define
	a base-10 to_chars overload for the uint128_t class type.
	* src/c++17/uint128_t.h: New file.
	* testsuite/20_util/to_chars/long_double.cc: No longer expect an
	execution FAIL on targets that have a large long double type
	but lack __int128.

0 files changed, 0 insertions, 0 deletions