efi_loader: detach runtime in ExitBootServices()

Linux can be called with a command line parameter efi=novamap, cf.
commit 4e46c2a95621 ("efi/arm/arm64: Allow SetVirtualAddressMap() to be
omitted"). In this case SetVirtualAddressMap() is not called after
ExitBootServices().

OpenBSD 32bit does not call SetVirtualAddressMap() either.

Runtime services must be set to an implementation supported at runtime
in ExitBootServices().

Reported-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Suggested-by: Alexander Graf <agraf@csgraf.de>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>