Update so that clean target in sys/boot will delete the symlink
created for machine

Reviewed by:	emaste
Add .NOPATH to the machine target so that builds in sys/boot work and don't
error out building zfs loader.

Submitted by:	sjg@freebsd.org
Deprecate the use of XDEV and XDEV_ARCH and just use TARGET/TARGET_ARCH
for the xdev build target, which is awesome and totally works.

Reapply svn R268377 with correct name of libsupc++ here as this does
resolve one dependancy race when building the xdev target.

the xdev target builds for amd64, i386, mips, mips64 and armv6 with this commit,
must be built as root, must be built from /usr/src, must not have a /usr/obj and
places the xdev tools in /usr/$TARGET_ARCH-freebsd

the xdev target still leaves some assorted files strewn about your /usr/src when
this is done and needs to be investigated further.

Phabric:	https://phabric.freebsd.org/D385
Submitted by:	bsdimp