Fix a hole in the GC generated for PutArgStructStk.

Fix a hole in the GC generated for PutArgStructStk.

On System V there is a hole (tanks @CarolEidt for pointing it out) where
while copying struct to the stack in the OutgoingParamArea the GC-ness
for the register used to copy the struct is not  properly set.
This change fixes the issue.

This change also makes sure when copying to/from stack, an emitAttr of
EA_PTRSIZE is used - stack variables are never moved by GC, so using this
type in such case should is fine.