exec_cmd: RUNTIME_PREFIX on z/OS systems

exec_cmd: RUNTIME_PREFIX on z/OS systems

Enable Git to resolve its own binary location using __getprogramdir
and getprogname.

Since /proc is not a mandatory filesystem on z/OS, we cannot rely on the
git_get_exec_path_procfs method to determine Git's executable path. To
address this, we have implemented git_get_exec_path_zos, which resolves
the executable path by extracting it from the current program's
directory and filename.

Signed-off-by: D Harithamma <harithamma.d@ibm.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>