[libc++] Reduces std::to_chars instantiations.

Instead of instantiating all functions called by std::to_chars for the
integral types only instantiate them for 32 and 64 bit integral types.
This results in a smaller binary when using different types.

In an example using the types: signed char, short, int, long, long long,
unsigned char, unsigned short, unsigned int, unsigned long, and
unsigned long long this saved 2792 bytes of code size. For libc++.so.1
is saves 688 bytes of code size (64-bit Linux).

This was discovered while investigating a solution for #52709.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D128215