When WITH_CLANG_BOOTSTRAP and WITHOUT_CLANG are both set, the
cross-tools stage does not build a cross clang binary. This is because
the Makefile in usr.bin/clang checks for WITHOUT_CLANG, and skips
building the binary.
To fix this, ensure that WITH_CLANG is set for the cross-tools phase
whenever WITH_CLANG_BOOTSTRAP is set. While here, skip using the
Makefile in usr.bin/clang, and directly use the Makefile in
usr.bin/clang/clang instead.
Reported by: avg
PR: 286154
MFC after: 1 week