meson: Add support for building efi binaries on multilib

meson: Add support for building efi binaries on multilib

This allows building 32bit versions of efi binaries on x86_64 machines
and vice-versa by passing "-Defi-cflags=-m32" to meson, provided the
32bit gnu-efi and gcc-multilib are available.

It is expected that distros that want to provide both ia32 and x64
versions to use a second build dir to build the non-native version
by adding -m32 to efi-cflags and then running the sd-boot/sd-stub
ninja target directly.