runtime: mark arm _lsvh nosplit (may fix arm build)

Mark the 386 one too for consistency,
although most of that code is no longer used.

TBR=dvyukov
CC=golang-dev
https://golang.org/cl/12227043

src/pkg/runtime/vlrt_386.c

@@ -423,6 +423,7 @@ _rshlv(Vlong *r, Vlong a, int b)
 	r->lo = (t << (32-b)) | (a.lo >> b);
 }
 
+#pragma textflag 7
 void
 _lshv(Vlong *r, Vlong a, int b)
 {

src/pkg/runtime/vlrt_arm.c

@@ -421,6 +421,7 @@ _rshlv(Vlong *r, Vlong a, int b)
 	r->lo = (t << (32-b)) | (a.lo >> b);
 }
 
+#pragma textflag 7
 void
 _lshv(Vlong *r, Vlong a, int b)
 {