Bugfix: Set output filename correctly.

Prior to this patch, whenever an different target compiler was used by
specifying the --macpo:compiler option, the output filename was not adjusted to
account for the "rose_" prefix inserted by the Rose compiler. Hence a command
invocation like: macpo.sh -c sample.c would create a file called rose_sample.o
instead of sample.o. This patch corrects the behaviour by creating a backup of
the original file, then renaming the rose_XXX.o file to XXX.o file. Thanks go
to Madhu from UTEP for pointing out the problem.