WITH_SYSTEM_COMPILER: Skip Clang/GCC bootstrap [Critical note for Toolchain changes]

In head is now the WITH_SYSTEM_COMPILER option.  It is off-by-default
for now but hopefully will be on-by-default relatively quickly.  It will
skip building the bootstrap Clang for
buildworld/kernel-toolchain/universe if /usr/bin/cc matches the tree.
It will also skip GCC if building for the same arch as the host system.
The Clang/GCC-to-by-installed will be still be built though.

See https://svnweb.freebsd.org/changeset/base/300354 for more details.

A critical note to toolchain developers, or anyone who touches the Clang
or GCC source files.  If you modify these files or add a new target
architecture into Clang, please bump the revision in the appropriate file:

Clang: lib/clang/include/clang/Basic/Version.inc FREEBSD_CC_VERSION
GCC: gnu/usr.bin/cc/cc_tools/freebsd-native.h FBSD_CC_VER

Still left todo:
 - Build one version of clang in universe if the WITH_SYSTEM_COMPILER
logic does not pass.

-- 
Regards,
Bryan Drewery