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.
(cherry picked from commit
86d821ddf5615e693ead667b2580898f46de8eb9)