Since we assume that GDB output is UTF-8, make sure it is so on all platforms.
Otherwise it'll choose some "appropriate" platform-specific default (e.g. CP1252 on Windows).
Un-disabled debug info tests on Windows.
auto merge of #8757 : vadimcn/rust/debug-info-tests, r=brson

Now that new LLVM has landed, the debug info works on Windows as well.   Most existing tests pass, except for the following four, which I left disabled for now:
lexical-scope-in-for-loop
lexical-scope-in-if
lexical-scope-in-match
lexical-scopes-in-block-expression

Also, fixed a small problem with the debug info test runner.