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
Document ISP 2532 support and hint.isp.0.vports tunable.

MFC after:	1 week