Reserving stack space on a per-argument basis

[Alexandre Oliva]

FUNCTION_ARG may return a PARALLEL containing a NULL_RTX, indicating
that a copy of an argument must be passed on the stack.  I'm working
on a port whose ABI requires stack space to be reserved for certain
types of arguments passed in registers.  In order to not waste the CPU
and memory cycles of creating a useless copy of such registers in
memory, I've come up with this patch.  Behavior is unchanged in ports
that don't use this new feature.  Ok to install?