libstdc++: Don't use std::tolower in <charconv> [PR103911]

As with std::isdigit in r12-6281-gc83ecfbe74a5cf, we shouldn't be using
std::tolower in <charconv> either.

	PR libstdc++/103911

libstdc++-v3/ChangeLog:

	* src/c++17/floating_from_chars.cc (find_end_of_float): Accept
	two delimeters for the exponent part in the form of a possibly
	NULL string of length two.  Don't use std::tolower.
	(pattern): Adjust calls to find_end_of_float accordingly.