[Libomptarget] Rename `libomptarget.rtl.x86_64` to `libomptarget.rtl.host` (#86868)

Summary:
All of these are functionally the same code, just compiled for separate
architectures. We currently do not expose a way to execute these on
separate architectures as the host plugin works using `dlopen` into the
same process, and therefore cannot possibly be an incompatible
architecture. (This could work with a remote plugin, but this is not
supported yet).

This patch simply renames all of these to the same thing so we no longer
need to check around for its varying definitions.