PPC startup code

[Roland McGrath]

> Given that the calling sequence is in fact defined in the ABI spec, I
> think we probably shouldn't change it without a really good reason.

This makes it sound like it's part of a formal spec, which it's not.  The
registers at startup are part of the ELF ABI spec.  But __libc_start_main
and its calling sequence are just part of the glibc ABI, which can and does
change between glibc releases.  There is just the set of practical issues I
mentioned with the part of the ABI involving crt1.o changing, which is rare
but not intrinsically different from other glibc ABI changes that are fine.