Address module reproducibility issues

Use .PATH & bare filename. This prevents the real source path from
being included in the built object, which improves reproducibility.

Signed-off-by: Dan McGregor <dan.mcgregor@usask.ca>
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1211
(cherry picked from commit 8c2f6c3be0125142d3c1782e4b0ee0634c584b9e)