[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

libunwind/test/libunwind/test/config.py

@@ -44,7 +44,8 @@ class Configuration(LibcxxConfiguration):
         # generated by default on all Targets.
         self.cxx.compile_flags += ['-funwind-tables']
         # Make symbols available in the tests.
-        if 'linux' in self.config.target_triple:
+        triple = self.get_lit_conf('target_triple', None)
+        if 'linux' in triple:
             self.cxx.link_flags += ['-Wl,--export-dynamic']
         if not self.get_lit_bool('enable_threads', True):
             self.cxx.compile_flags += ['-D_LIBUNWIND_HAS_NO_THREADS']