[SystemZ][z/OS][libcxx]: fix libcxx test cases failed on ebcdic mode on z/OS

This patch is to fix 2 libcxx test cases, test cases assumed 'a' > 'A' which is not case in z/OS platform on ebcdic mode, modified test cases to compare between upper letters or lower letters, or digits so ordering will be true for all platform.

Differential Revision: https://reviews.llvm.org/D104748