runtime: mark functions as static where possible

Update #8092

LGTM=dvyukov
R=golang-codereviews, minux, dvyukov
CC=golang-codereviews
https://golang.org/cl/122250043

src/pkg/runtime/cpuprof.goc

@@ -314,7 +314,7 @@ flushlog(Profile *p)
 
 // getprofile blocks until the next block of profiling data is available
 // and returns it as a []byte.  It is called from the writing goroutine.
-Slice
+static Slice
 getprofile(Profile *p)
 {
 	uint32 i, j, n;

src/pkg/runtime/mheap.c

@@ -211,7 +211,7 @@ mheap_alloc(MHeap *h, uintptr npage, int32 sizeclass, bool large)
 	return s;
 }
 
-void
+static void
 mheap_alloc_m(G *gp)
 {
 	MHeap *h;

src/pkg/runtime/proc.c

@@ -1384,7 +1384,7 @@ top:
 // appropriate time. After calling dropg and arranging for gp to be
 // readied later, the caller can do other work but eventually should
 // call schedule to restart the scheduling of goroutines on this m.
-void
+static void
 dropg(void)
 {
 	if(g->m->lockedg == nil) {