Support for cross compiled C to native CMS and a C library.

Except for the added makefile an unmodified version of zlib-1.2.8 is cross compiled to zlib.txtlib.

[gas2asm] (https://github.com/jphartmann/gas2asm) converts the -S output from gcc to HLASM input. Required.

[txtlib] (https://github.com/jphartmann/CMS-TXTLIB) generates the target libraries when object decks can be created at the workstation. OPtional.