In preparation for handling more binaries than just cc, handle
the case of "probe_target_compiler $cpu" directly in the function,
setting the target_* variables based on the ones that are used to
build QEMU.  The clang check also needs to be moved after this
fallback.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220517092616.1272238-10-pbonzini@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20220527153603.887929-21-alex.bennee@linaro.org>