OptionalObsoleteFiles: move /usr/bin/CC to MK_TOOLCHAIN section

/usr/bin/CC is installed by usr.bin/clang/clang/Makefile, as with
/usr/bin/cc, /usr/bin/cpp, etc., and is not controlled by MK_CXX.
Move it to the same section as those tools.

(It may be that these should all be under
MK_TOOLCHAIN == no || MK_CLANG_IS_CC == no, but that seems like
unnecessary complexity.)

Sponsored by: The FreeBSD Foundation

(cherry picked from commit f7ea22e2115329b7a4f2c6620e59e644f509a4ca)