configure: cross-compiling with empty cross_prefix

The iOS toolchain does not use the host prefix naming convention. So we
need to enable cross-compile options while allowing the PREFIX to be
blank.

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Joelle van Dyne <j@getutm.app>
Message-id: 20210126012457.39046-3-j@getutm.app
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>