[SystemZ][z/OS] Fix build errors on z/OS in the Unix .inc files (#74758)

This patch resolves the following errors on z/OS:

error: no member named 'wait4' in the global namespace
error: no member named 'ru_maxrss' in 'rusage'
error: use of undeclared identifier 'strsignal'
error: Cannot get usage times on this platform
error: Cannot get malloc info on this platform