configure: unify creation of cross-compilation Makefiles

Let write_target_makefile handle both host and container cross compilers.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20220929114231.583801-27-alex.bennee@linaro.org>