[SystemZ][z/OS] Fix filesystem testing

This patch resolves many of the failures in the `filesystems/` buckets in the libc++ tests. It adds the correct flag to `fopen` and marks a test case as unsupported. In particular, that test assumes time is stored as a 64 bit value when on MVS it is stored as 32 bit.

Differential Revision: https://reviews.llvm.org/D113298