efi_selftest: on sandbox use host specific assembly

The selftest checking the handling of exceptions in UEFI binaries is using
assembly to provide an undefined instruction. On the sandbox the correct
form of the instruction depends on the host architecture.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>