runtime: move stubs.goc code into runtime.c

Now that the calling conventions are the same,
there's no danger to using plain C for these.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=dvyukov, golang-codereviews, iant, khr, r
https://codereview.appspot.com/134580043
runtime/cgo: use just #include "textflag.h"

This will withstand the src/pkg/ to src/ move.

LGTM=r
R=iant, r
CC=golang-codereviews
https://codereview.appspot.com/134530044
cmd/yacc: move cmd/yacc/expr to cmd/yacc/testdata/expr

This will keep the go command from trying to build it
when the cmd/ tree is no longer a special case.

Also update doc.go to refer to the correct location.
(It was incorrect even before this CL.)

LGTM=r
R=iant, r
CC=golang-codereviews
https://codereview.appspot.com/134560043