implement inline 5- and 6-argument syscalls for mipsn32 and mips64

implement inline 5- and 6-argument syscalls for mipsn32 and mips64

n32 and n64 ABIs add new argument registers vs o32, so that passing on
the stack is not necessary, so it's not clear why the 5- and
6-argument versions were special-cased to begin with; it seems to have
been pattern-copying from arch/mips (o32).

i've treated the new argument registers like the first 4 in terms of
clobber status (non-clobbered). hopefully this is correct.