Use LOCAL_MODULE_STEM_32 and LOCAL_MODULE_STEM_64 to build
linker and linker64. Set LOCAL_MULTILIB := both for consistency
with other rules of this type, although LOCAL_MULTILIB := both is
only consumed by executable.mk which is skipped by
linker_executable.mk. Duplicate the necessary new parts of
executable.mk.
Change-Id: I888e87cf125cfbcfdee6a55b6377d8bc5ce402ea