libstdc++: Another merge from fast_float upstream [PR107468]

libstdc++: Another merge from fast_float upstream [PR107468]

Upstream fast_float came up with a cheaper test for
fegetround () == FE_TONEAREST using one float addition, one subtraction
and one comparison.  If we know we are rounding to nearest, we can use
fast path in more cases as before.
The following patch merges those changes into libstdc++.

2022-11-24  Jakub Jelinek  <jakub@redhat.com>

	PR libstdc++/107468
	* src/c++17/fast_float/MERGE: Adjust for merge from upstream.
	* src/c++17/fast_float/fast_float.h: Merge from fast_float
	2ef9abbcf6a11958b6fa685a89d0150022e82e78 commit.