Point P_tmpdir to /tmp instead of /var/tmp.

Point P_tmpdir to /tmp instead of /var/tmp.

Temp files are much better of placed in /tmp than in the not-so-volatile
/var/tmp.  Additionally, people might get space issues or might have tuned
their /tmp for speed, but not /var/tmp.

Libiberty, among other consumers is using P_tmpdir to place the temp files,
leading compiler temp files being created in /var/tmp.