[clang] [SystemZ/[z/OS] Guard source-epoch related testcase to 64-bit clang targets (PR #210341)

[Abhina Sree via cfe-commits]

abhina-sree wrote:

> clang-target-64-bits isn't precisely the correct condition, but close enough, I guess. For reference, Ubuntu 24.04 has 64-bit time_t on 32-bit targets, but the buildbot in question is running 22.04.

Is there a better check we can use?

https://github.com/llvm/llvm-project/pull/210341