libstdc++: Make __from_chars_alnum_to_val conversion explicit

libstdc++: Make __from_chars_alnum_to_val conversion explicit

The optimizations from commit r12-8175-ga54137c88061c7 introduced a
clang integer sanitizer error.

Fix this with an explicit static_cast, similar to the fix for PR 96766.

libstdc++-v3/ChangeLog:

	* include/std/charconv (__from_chars_alnum_to_val): Replace
	implicit conversion from int to unsigned char with explicit
	cast.

(cherry picked from commit 20ab3972240aff596a3fa98e9fb09ddc0658fbb3)