cmd/compile: remove overflow pointer padding for nacl

cmd/compile: remove overflow pointer padding for nacl

CL 200077 removed nacl bits in the toolchain, but it misses the code to
add pointer overflow padding, which is specific for nacl.

This CL removes that part.

Passes toolstash-check.

Updates #30439

Change-Id: I1e77cade9f31690e16cd13d3445a98b500671252
Reviewed-on: https://go-review.googlesource.com/c/go/+/202159
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>