I changed all the NACL_SYSJMP to NACL_SYSCALL in
an earlier CL, but I missed the fact that NACL_SYSCALL
will push another return PC on the stack, so that the
arguments will no longer be in the right place.
Since we have to make our own call, we also have to
copy the arguments. Do that.

Fixes nacl/386 build.

TBR=minux
CC=golang-codereviews
https://golang.org/cl/135050044