cmd/cgo, cmd/go, go/build: sort flag lists

CFLAGS comes before CPPFLAGS.
Also fix one typo CPPCFLAGS.

Cleanup for CL 8248043.

R=golang-dev, iant, alberto.garcia.hierro
CC=golang-dev
https://codereview.appspot.com/9965045
runtime: fix comments (g->gobuf became g->sched long ago)

Should reduce size of CL 9868044.

R=golang-dev, ality
CC=golang-dev
https://codereview.appspot.com/10045043