Add support for an external cross compiler.  The cross compiler is
specified by passing the XCC, XCXX, and XCPP variables (corresponding to
CC, CXX, and CPP) to buildworld/buildkernel.  The compiler must be clang
or be configured to target the appropriate architecture.

To speed build times, if XCC is an absolute path or
WITHOUT_CROSS_COMPILER is defined then no cross compiler will be built
during the cross-tools stage.

Limited documentation of this feature can currently be found at:

https://wiki.freebsd.org/ExternalToolchain

This functionality should be considered experimental and is subject to
change without notice.

Sponsored by:	DARPA, AFRL
Discussed with:	imp, sjg
Document WITHOUT_CROSS_COMPILER and WITHOUT_FORMAT_EXTENSIONS.