Add OBJDUMP to sys.mk. The EFI loader uses it however without this it can
be set but empty.

Obtained from:	ABT Systems
Sponsored by:	The FreeBSD Foundation


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274503 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f