[PATCH]Fix GuessOS to work correctly for both TPF and OS/390

[Ovies Brabson <ov...@us.ibm.com>]

Joe Moenich from the TPF development area at IBM contacted me and told
me that GuessOS was no longer working correctly after the OS/390 port
patch was committed.  They sometimes compile for TPF on OS/390 and in
that case GuessOS returns OS/390 as the platform.  This happens becase
the case statement that determines the OS is before the code that
handles the exception OSes.  When TPF is compiled on OS/390, the case
for OS/390 is matched before the TPF environment variable can be
checked.  I moved the check for the TPF environment variable to the top
of the script.  I sent the change to Joe and he tried it on TPF and it
works.  I also verified it on my OS/390 system here.  Please review.

Thanks.