selftests/nolibc: allow building i386 with multiarch compiler

When building with a multiarch-capable compiler, like those provided by
common distributions the -m32 argument is required to build 32bit code.

Wrap it in cc-option in case the compiler is not multiarch-capable.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20230917-nolibc-syscall-nr-v2-1-03863d509b9a@weissschuh.net