Fix build with WITH_CLANG_BOOTSTRAP and WITHOUT_CLANG

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.

PR: 286154
Reported by: avg
Reviewed by: avg, emaste
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D49886

(cherry picked from commit ea231471d024e93279dc2196d6d5d87e199ad55b)