MFC r236528:

MFC r236528:

During buildworld and buildkernel, define EARLY_BUILD in the earlier
stages (build-tools, cross-tools, etc) of the build, so we can detect in
bsd.*.mk whether to pass compiler-specific flags to ${CC}.

In particular, this commit will allow using WITH_CLANG_IS_CC when the
base compiler is still gcc, and when ${CC}, ${CXX} and ${CPP} are left
at their defaults.  The early stages will then be built using gcc, and
no clang-specific flags will be passed to it.  The later stages will be
built as usual.

The EARLY_BUILD define can also serve other uses, such as building the
world stage C++ executables with libc++ instead of libstdc++: during the
early build stages, we cannot assume libc++ is already available, so we
must still build with libstdc++ at that time.

git-svn-id: svn://svn.freebsd.org/base/stable/9@236851 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f