[libcxx] Fix exception raised during downstream bare-metal libunwind tests

Fix for the following exception.

AttributeError: 'TestingConfig' object has no attribute 'target_triple'

Related revision: https://reviews.llvm.org/D102012

'TestingConfig' object has no attribute 'target_triple'

Reviewed By: #libunwind, miyuki, danielkiss, mstorsjo

Differential Revision: https://reviews.llvm.org/D103140