libstdc++: Apply modifications to our local copy of fast_float

This performs the following modifications to our local copy of fast_float
in order to make it more readily usable in our std::from_chars
implementation:

  * Remove system #includes
  * Replace stray call to assert
  * Use the standard chars_format and from_chars_result types

libstdc++-v3/ChangeLog:

	* src/c++17/fast_float/LOCAL_PATCHES: Update.
	* src/c++17/fast_float/fast_float.h: Apply local modifications.