Libgcc symbols

[Richard Kenner]

    Oh, I didn't realize you were talking specifically about MIPS.  I
    suppose GP may have to be stored in the stack frame somewhere, by
    builtin_setjmp_setup or the function prologue, so that
    builtin_setjmp_receiver can restore it.  But then, I know very little
    of MIPS calling conventions, so there may even be a reason why this
    would be impossible to do.

At the time I added _dummy, I was told it could not be done, but I forget
exactly why.  The suggestion was to pass the address of a function and call
it since the return from that call would set GP.