cmd/cgo, cmd/ld: new cgo object file section

Switch to new pragma names, but leave old ones available for now.
Merge the three cgo-related sections in the .6 files into a single
cgo section.

R=golang-dev, iant, ality
CC=golang-dev
https://codereview.appspot.com/7424048
runtime/cgo: fix arm build

During my global search and replace I forgot to open *.S (capital).

R=golang-dev
TBR=golang-dev
CC=golang-dev
https://codereview.appspot.com/7415047