libstdc++: Import the fast_float library

We're going to use the fast_float library[1] in our (compiled-in)
floating-point std::from_chars implementation for faster and more
portable parsing of binary32/64 decimal strings.

The single file fast_float.h is an amalgamation of the entire library,
which can be (re)generated with the amalgamate.py script (from the
fast_float repository) via the command

  python3 ./script/amalgamate.py --license=MIT \
    > $GCC_SRC/libstdc++-v3/c++17/fast_float/fast_float.h

The code has a GPL-compatible license.

[1]: https://github.com/fastfloat/fast_float



libstdc++-v3/ChangeLog:

	* src/c++17/fast_float/LOCAL_PATCHES: New file.
	* src/c++17/fast_float/MERGE: New file.
	* src/c++17/fast_float/README.md: New file, copied from the
	fast_float repository.
	* src/c++17/fast_float/fast_float.h: New file, an amalgamation
	of the fast_float library.

Signed-off-by: Patrick Palka <ppalka@redhat.com>