dex2oat: Avoid extra std::string copy

The function Fatal() takes a const std::string reference, so it's
inefficient to pass it std::string::c_str().  While it's not
useful optimize a fatal code path, we still fix this so static
analysis tools finding this issue in other parts of the code
are less noisy.

Test: TreeHugger
Change-Id: I42231b40c1af2f866c451e169f2d8362154853b0