os: in Getwd, $PWD override syscall.Getwd

This makes os.Getwd mimic C getwd on OS X,
and possibly other systems. The change on OS X
was a regression from 1.2 to 1.3.

Fixes issue 8400.

LGTM=bradfitz
R=iant, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/118970043
cmd/cc, runtime: eliminate use of the unnamed substructure C extension

Eliminating use of this extension makes it easier to port the Go runtime
to other compilers. This CL also disables the extension in cc to prevent
accidental use.

LGTM=rsc, khr
R=rsc, aram, khr, dvyukov
CC=axwalk, golang-codereviews
https://codereview.appspot.com/106790044

Committer: Russ Cox <rsc@golang.org>