runtime: fixes for -buildmode=c-archive

    
    With -buildmode=c-archive, initsig is called before the memory
    allocator has been initialized.  The code was doing a memory
    allocation because of the call to funcPC(sigtramp).  When escape
    analysis is fully implemented, that call should not allocate.  For
    now, finesse the issue by calling a C function to get the C function
    pointer value of sigtramp.
    
    When returning from a call from C to a Go function, a deferred
    function is run to go back to syscall mode.  When the call occurs on a
    non-Go thread, that call sets g to nil, making it impossible to add
    the _defer struct back to the pool.  Just drop it and let the garbage
    collector clean it up.
    
    Reviewed-on: https://go-review.googlesource.com/33675

From-SVN: r242992