Code under arch/sandbox/os is meant to link against the libc primitives
of the OS, not the barebox counterparts (e.g. for malloc).

54d47eb7e912 ("sandbox: support forcing 32-bit x86") broke this
rendering the sandbox architecture unusable. Fix this.

Reported-by: Peter Mamonov <pmamonov@gmail.com>
Fixes: 54d47eb7e912 ("sandbox: support forcing 32-bit x86")
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>