Teach pmcannotate about $TMPDIR and _PATH_TMP

Convert pmcannotate to using $TMPDIR and _PATH_TMP rather than hard
coding /tmp for temporary files. Pmcannotate sometimes needs quite a
lot of space to store the output from objdump, and will fail in odd
ways if that output is truncated due to lack of space in /tmp.

Reviewed by: jtl
Sponsored by: Netflix