Build infrastructure for elftoolchain tools

Set WITH_ELFTOOLCHAIN_TOOLS in src.conf to use the elftoolchain version
of the following tools:

• addr2line
• elfcopy (strip / mcs)
• nm
• size
• strings

Reviewed by: bapt (earlier version)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D1224