x86_64: Fix indirection in struct paramaters

x86_64: Fix indirection in struct paramaters

The first loop setting up struct arguments must not remove
elements from the vstack (via vtop--), as gen_reg needs them to
potentially evict some argument still held in registers to stack.

Swapping the arg in question to top (and back to its place) also
simplifies the vstore call itself, as not funny save/restore
or some "non-existing" stack elements need to be done.

Generally for a stack a vop-- operation conceptually clobbers
that element, so further references to it aren't allowed anymore.