[libc++] Make to_chars base 10 header only.

The functions to_chars and from_chars should offer 128-bit support. This
is the first step to implement 128-bit version of to_chars. Before
implementing 128-bit support the current code will be polished.

This moves the code from the dylib to the header in prepartion of

P2291 "Add Constexpr Modifiers to Functions to_chars and from_chars for
Integral Types in <charconv> Header"

Note some more cleanups will be done in follow-up commits
- Remove the _LIBCPP_AVAILABILITY_TO_CHARS from to_chars. With all code
  in the header the availablilty macro is no longer needed. This
  requires enabling the unit tests for additional platforms.
- The code in the dylib can switch to using the header implementation.
  This allows removing the code duplicated in the header and the dylib.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D125704