configure: create native file with contents of $host_cc

The argument of --host-cc is not obeyed when cross compiling.  To avoid
this issue, place it in a configuration file and pass it to meson
with --native-file.

While at it, clarify that --host-cc is not obeyed anyway when _not_
cross compiling, because cc="$host_cc" is placed before --host-cc is
processed.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>