efi_selftest: allow building on ARMv7-M

ARMv7-M only supports the Thumb instruction set. Our current crt0 code does
not support it. With the patch we can build all unit tests of the EFI
subsystem that do not require crt0.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>