[SystemZ][z/OS] Save strings for CC_PRINT env vars

The contents of the string returned by getenv() is not guaranteed across calls to getenv(). The code to handle the CC_PRINT etc env vars calls getenv() and saves the results in just a char *. The string returned by getenv() needs to be copied and saved. Switching the type of the strings from char * to std::string will do this and manage the alloated memory.

Differential Revision: https://reviews.llvm.org/D98554