Create the native-xtools target. This target creates only the cross

Create the native-xtools target. This target creates only the cross
building toolchain for the host computer. This toolchain produces
TARGET_ARCH and assumes the rest of the system contains libraries for
the target. It is intended to be used in a "qemu-user jail" where all
the binaries would otherwise be the target architecture's to build
ports. However, emulation of the compilers is too slow, so we build
native binaries for that. Rather than use the xdev produced binaries,
with all their weird links and paths, these binaries use the native
paths. They will not work unless installed into the qemu-user jail.

Differential Revision: https://phabric.freebsd.org/D518
Reviewed by: sbruno@