upc2c supresses the contribution of certain upc header files
in the translated C file (e.g. upc.h, ..).  In the past we
had a list of files we suppress the output from, but we did
not differentiate if the files are system headers or files
on the user specified path.

This change makes sure we suppress the output only from the
system file (included with -isystem).  This requires a change
in the upcc driver where UPCR headers directory is included
with "-isystem".