Some existing cgen-based sid ports (arm7t, for example) may run into
problems in case you run make cgen-all -jN with some N>1, because
CGEN.sh reuses the same temporary file names for concurrent builds.
Oops.  This patch fixes this problem, except on filesystems that have
limited filenames, in which case the PID trailer is likely to be
dropped, so it will hopefully still work without -j on such old,
broken systems.  I'm checking this in, approved by Frank.