runtime: print C functions in traceback

    
    Since gccgo can trace back through C code as easily as Go code, we
    should print C functions in the traceback.
    
    This worked before https://golang.org/cl/31230 for a dumb reason.  The
    default value for runtime.traceback_cache was, and is, 2 << 2, meaning
    to print all functions.  The old C code for runtime_parsedebugvars
    would return immediately and do nothing if the environment variable
    GODEBUG was not set (if GODEBUG was set it would later call
    setTraceback.  The new Go code for runtime.parsedebugvars does not
    return immediately if GODEBUG is not set, and always calls
    setTraceback.  Either way, if GOTRACEBACK is not set, setTraceback
    would set traceback_cache to 1 << 2, meaning to only print non-runtime
    functions and having the effect of not printing plain C functions.
    
    Keep the current handling of GODEBUG/GOTRACEBACK, which matches the gc
    library, but add an extra check to print C functions by default.
    
    Reviewed-on: https://go-review.googlesource.com/33717

From-SVN: r243083