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

[Sean Perry via cfe-commits]

perry-ca wrote:

Should check out clang/test/Preprocessor/SOURCE_DATE_EPOCH.c too.  It uses clang-target-64-bits for the same purpose.  Any changes should be applied there too.

Should we introduce a `system-64-bits` feature?  Technically, the dependency isn't on whether the target triple is 64-bit but whether the time_t in clang is 64-bit.

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