Observe $TMPDIR variable when creating tmp files

Observe $TMPDIR variable when creating tmp files

POSIX defines $TMPDIR as containing the pathname of the directory where
programs can create temporary files. On most systems, this variable points to
"/tmp". However, on some systems it can point to a different location.
Without respecting this variable, yosys fails to run on such systems.

Signed-off-by: Mohamed A. Bamakhrama <mohamed@alumni.tum.de>