DWARF generation appears to assume Cpos is cheap and this makes linking godoc
about 8% faster and linking the standard library into a single shared library
about 22% faster on my machine.

Updates #10571

Change-Id: I3f81efd0174e356716e7971c4f59810b72378177
Reviewed-on: https://go-review.googlesource.com/9913Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>