[rfa] mips argument passing fixes for o32

[Andrew Cagney]

FYI,

> These are based on testsuite failures (call-*-st, if I remember correctly)
> and reading the argument passing code in GCC.  The struct alignment fix
> definitely agrees with the ABI, though it's not always clear on this point -
> it becomes necessary for us when the return value is a struct and thus there
> is a hidden pointer as the first argument.  The shift fix matches this
> comment in GCC and is not really specified by the ABI document:


Reviewing this could take a long time.

	Andrew