runtime: replace unions with structs

Unions can break precise GC.
Update #5193.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/8456043

src/pkg/runtime/atomic_arm.c

@@ -5,9 +5,9 @@
 #include "runtime.h"
 #include "arch_GOARCH.h"
 
-static union {
+static struct {
 	Lock l;
-	byte pad [CacheLineSize];
+	byte pad[CacheLineSize-sizeof(Lock)];
 } locktab[57];
 
 #define LOCK(addr) (&locktab[((uintptr)(addr)>>3)%nelem(locktab)].l)

src/pkg/runtime/sema.goc

@@ -44,13 +44,13 @@ struct SemaRoot
 // Prime to not correlate with any user patterns.
 #define SEMTABLESZ 251
 
-union semtable
+struct semtable
 {
 	SemaRoot;
-	uint8 pad[CacheLineSize];
+	uint8 pad[CacheLineSize-sizeof(SemaRoot)];
 };
 #pragma dataflag 16 /* mark semtable as 'no pointers', hiding from garbage collector */
-static union semtable semtable[SEMTABLESZ];
+static struct semtable semtable[SEMTABLESZ];
 
 static SemaRoot*
 semroot(uint32 *addr)