efi_loader: correctly identify binary name

Only on the sandbox the default EFI binary name (e.g. BOOTX64.EFI) must
match the host architecture.

In all other cases we must use the target architecture.

Use #elif where appropriate.

Reported-by: Vagrant Cascadian <vagrant@reproducible-builds.org>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>