[OpenEmbedded] Explicitly specify -rtlib in tests
Tests added in r338294 implicitly assume that libgcc is the runtime library,
but that's not the case when the user configures Clang to use compiler-rt in
which case these tests will break. Explicitly request libgcc when invoking
clang in these tests to avoid that.
Differential Revision: https://reviews.llvm.org/D50123