proc: apply and correct build constraint

It must occur in the first few lines of the file. Also the comma is
incorrect as it's treated as logical AND. However we want to allow
building on amd64 and i386.

proc.go

+// endianness!! (/proc/net/tcp "parsing")
+// +build amd64 386
+
 /*
 MIT License
 
@@ -23,8 +26,6 @@ OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 SOFTWARE.
 */
 
-// endianness!! (/proc/net/tcp "parsing")
-// +build amd64, 386
 package main
 
 import (