[libc++] Implements 128-bit support in to_chars.

This is required by the Standard and makes it possible to add full
128-bit support to format.

The patch also fixes 128-bit from_chars "support". One unit test
required a too large value, this failed on 128-bit; the fix was to add
more characters to the input.

Note only base 10 has been optimized. Other bases can be optimized.

Note the 128-bit lookup table could be made smaller. This will be done later. I
really want to get 128-bit working in to_chars and format in the upcomming
LLVM 15 release, these optimizations aren't critical.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D128929