cmd/gc: say 'non-constant array bound' instead of 'invalid array bound'

Fixes issue 8196.

LGTM=adonovan
R=adonovan
CC=golang-codereviews
https://codereview.appspot.com/141510044
runtime: remove duplicated Go constants

The C header files are the single point of truth:
every C enum constant Foo is available to Go as _Foo.
Remove or redirect duplicate Go declarations so they
cannot be out of sync.

Eventually we will need to put constants in Go, but for now having
them be out of sync with C is too risky. These predate the build
support for auto-generating Go constants from the C definitions.

LGTM=iant
R=iant
CC=golang-codereviews
https://codereview.appspot.com/141510043