runtime: get missing function name from symbol table

runtime: get missing function name from symbol table

    If we trace back through code that has no debug info, as when calling
    through C code compiled with -g0, we won't have a function name.
    Try to fetch the function name using the symbol table.

    Adding the test case revealed that gotest failed to use the gccgo tag
    when matching files, so add that.

    Reviewed-on: https://go-review.googlesource.com/92756

From-SVN: r257495