[SystemZ][z/OS] ASCII/EBCDIC support with no coexistence

The aim of this patch is to break up the larger patch (https://reviews.llvm.org/D111323) to be more upstream friendly. In particular, this patch adds the char encoding sensitive changes but does not use inline namespaces as before. The use of namespaces to build both versions of the library, and localization of error messages will follow in a subsequent patch.

Differential Revision: https://reviews.llvm.org/D114813