/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.)

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