Stop unwinding on undefined return address

Milian Wolff <mail@milianw.de> reported that this greatly
improved unwinding perf on some his apps.
Add TileGx platform support to libunwind.
"make check" passed.
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All 34 tests behaved as expected (2 expected failures)
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Zhi-Gang Liu @ Tilera
Merge https://github.com/zliu2014/libunwind-tilegx