If gcc supports multiple architectures, barebox uses only the default
when compiling for ARCH=sandbox, this is e.g. the case with compilers
that generate both 32- and 64-bit x86 executables.

There can be good reasons to force 32-bit though, e.g. to reduce memory
consumption while fuzzing or to temporarily avoid 32-to-64-bit warnings,
while running the static analyzer. Add an option for this.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>