Actually assign arguments to r0-r3 at the call site. Previously
this was left unhandled. There was only logic for pulling
formal parameters out of r0-r3.

Refactor the GPR counter and move it into a class so that the
rounding up for i64 arguments is in one place for callsites
and for pulling out of parameters. We might be able to use a
similar pattern to count the FP/SIMD registers later.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4076
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1187513006.