libstdc++: Avoid ASCII assumptions in floating_from_chars.cc

In starts_with_ci and in __floating_from_chars_hex's inf/nan handling,
we were assuming that the letters are contiguous and that 'A' + 32 == 'a'
which is true for ASCII but not for other character encodings.

This patch fixes starts_with_ci by using a constexpr lookup table that
maps uppercase letters to lowercase, and fixes __floating_from_chars_hex
by using __from_chars_alnum_to_val.

libstdc++-v3/ChangeLog:

	* include/std/charconv (__from_chars_alnum_to_val_table):
	Simplify initialization of __lower/__upper_letters.
	(__from_chars_alnum_to_val): Default the template parameter to
	false.
	* src/c++17/floating_from_chars.cc (starts_with_ci): Don't
	assume the uppercase and lowercase letters are contiguous.
	(__floating_from_chars_hex): Likewise.