ObjectTriple isn't used since f1ec44e2. Delete
it, and simplify CEntryStub on all backends.

Bug: 
Change-Id: I046525afceb25b484fd96c7ee81c73fb03168ca0
Reviewed-on: https://chromium-review.googlesource.com/704858Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Martyn Capewell <martyn.capewell@arm.com>
Cr-Commit-Position: refs/heads/master@{#48369}