all: move //go: function directives directly above functions

These directives affect the next declaration, so the existing form is
valid, but can be confusing because it is easy to miss. Move then
directly above the declaration for improved readability.

CL 69120 previously moved the Gosched nosplit away to hide it from
documentation. Since CL 224737, directives are automatically excluded
from documentation.

Change-Id: I8ebf2d47fbb5e77c6f40ed8afdf79eaa4f4e335e
Reviewed-on: https://go-review.googlesource.com/c/go/+/472957


Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>