cmd/internal/gc: fix build on big endian systems

cmd/internal/gc: fix build on big endian systems

The siz argument to both runtime.newproc and runtime.deferproc is
int32, not uintptr. This problem won't manifest on little-endian
systems because that stack slot is uintptr sized anyway. However,
on big-endian systems, it will make a difference.

Change-Id: I2351d1ec81839abe25375cff95e327b80764c2b5
Reviewed-on: https://go-review.googlesource.com/9647
Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>