[libc++] Don't use static constexpr in headers.

This was noticed in the review of D125704. In that commit only the new
table has been adapted. This adapts the existing tables.

Note since libc++'s charconv is backported to C++11 it's not possible to
use inline constexpr variables. The were introduced in C++17.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D126887