Ensure backends available in 'opt' are also available in 'bugpoint'

This patch links LLVM back-ends into bugpoint the same way they are already
available in 'opt' and 'clang'. This resolves an inconsistency that allowed the
use of LLVM backends in loadable modules that run in 'opt', but that would
prevent the debugging of these modules with bugpoint due to unavailable /
unresolved symbols.

For e.g. In D31859, Polly requires the NVPTX back-end.

Reviewers: hfinkel, bogner, chandlerc, grosser, Meinersbur

Subscribers: bollu, mgorny, grosser, Meinersbur

Tags: #polly

Contributed by: Singapuram Sanjay

Differential Revision: https://reviews.llvm.org/D32003

llvm-svn: 306208