runtime: prevent deadlock when profiling signal arrives in stack scan

    
    Precise stack scan needs to unwind the stack. When it is
    unwinding the stack, if a profiling signal arrives, which also
    does a traceback, it may deadlock in dl_iterate_phdr. Prevent
    this deadlock by setting up runtime_in_callers before traceback.
    
    Reviewed-on: https://go-review.googlesource.com/c/155766

From-SVN: r267457