elftoolchain: stop leaving tempfiles on error

Temporary files were not cleaned up, resulting in $TMPDIR or even
the current directory becoming littered with ecp.* files.

This happened with error and even sometimes on success!

Approved by: dim
MFC after: 4 weeks
Accepted upstream: https://sourceforge.net/p/elftoolchain/code/3918/
Differential Revision: https://reviews.freebsd.org/D28651

(cherry picked from commit 5ac70383c8b32eeec80426e837960977971c7c2b)