Add an option similar to the -qtarget option in XL to allow the user to
say they want to be able to run the generated program on an older
version of the LE environment. This option will do two things:
- set the `__TARGET_LIBS` macro so the system headers exclude newer
interfaces when targeting older environments
- set the arch level to match the minimum arch level for that older
version of LE. It doesn't happen right now since all of the supported LE
versions have a the same minimum ach level. So the option doesn't change
this yet.
The user can specify three different kinds of arguments:
1. -mzos-target=zosv*V*r*R* - where V & R are the version and release
2. -mzos-target=0x4vrrmmmm - v, r, m, p are the hex values for the
version, release, and modlevel
3. -mzos-target=current - uses the latest version of LE the system
headers have support for
