cgo: disallow use of C.errno
cgo: disallow use of C.errno

It's incorrect, and it causes cryptic linker errors.

Fixes issue 1360.

R=r, r2
CC=golang-dev
http://codereview.appspot.com/3880041

apparentlymart/golang,ssrl/go,apparentlymart/golang,apparentlymart/golang,ssrl/go,gnanderson/go,gnanderson/go,pombredanne/go-deleteme,gnanderson/go,gnanderson/go,dsriram/golang,dsriram/golang,ssrl/go,gnanderson/go,ssrl/go,ssrl/go,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,tv42/old-go,dsriram/golang,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,dsriram/golang,dsriram/golang,tv42/old-go,pombredanne/go-deleteme,ssrl/go,pombredanne/go-deleteme,gnanderson/go,gnanderson/go,tv42/old-go,ssrl/go,tv42/old-go,dsriram/golang,apparentlymart/golang,pombredanne/go-deleteme,dsriram/golang,tv42/old-go,pombredanne/go-deleteme