libstdc++: Update from latest fast_float [PR107468]

libstdc++: Update from latest fast_float [PR107468]

The following patch partially updates from fast_float trunk.
That way it gets fix for the incorrect rounding case,
PR107468 aka https://github.com/fastfloat/fast_float/issues/149
Using std::fegetround showed in benchmarks too slow, so instead of
doing that the patch limits the fast path where it uses floating
point multiplication rather than integral to cases where we can
prove there will be no rounding (the multiplication will be exact, not
just that the two multiplication or division operation arguments are
exactly representable).

2022-11-07  Jakub Jelinek  <jakub@redhat.com>

	PR libstdc++/107468
	* src/c++17/fast_float/fast_float.h: Partial merge from fast_float
	662497742fea7055f0e0ee27e5a7ddc382c2c38e commit.
	* testsuite/20_util/from_chars/pr107468.cc: New test.

(cherry picked from commit cb0ceeaee9e041aaac3edd089b07b439621d0f29)