Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

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