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

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

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

  * Remove all #includes
  * Remove copy_special_str routines
  * Adjust the exponent formatting to match printf
  * Remove some functions we're not going to use
  * Add an out-parameter to d2exp_buffered_n for the scientific exponent
  * Store the sign bit inside struct floating_decimal_[32|64]
  * Rename [df]2s_buffered_n and change their return type
  * Make generic_binary_to_decimal take the bit representation in parts

libstdc++-v3/ChangeLog:

	* src/c++17/ryu/common.h, src/c++17/ryu/d2fixed.c,
	src/c++17/ryu/d2fixed_full_table.h, src/c++17/ryu/d2s.c,
	src/c++17/ryu/d2s_intrinsics.h, src/c++17/ryu/f2s.c,
	src/c++17/ryu/f2s_intrinsics.h, src/c++17/ryu/generic_128.c:
	Apply local modifications.