efi_loader: clean up runtime detaching

The detaching of the runtime will have to move to ExitBootServices() to
encompass operating system that do not call SetVirtualAddressMap().

This patch changes the logic for the relocation of the pointers in the
runtime table such that the relocation becomes independent of the entries
in the detach list.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>