runtime: give an explicit name to profiling Bucket sub-structs.

Fixes compilation of runtime on Solaris where the inner struct
was not called "_4_".

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/129460043

src/pkg/runtime/mprof.go

@@ -122,7 +122,7 @@ func BlockProfile(p []BlockProfileRecord) (n int, ok bool) {
 		ok = true
 		idx := 0
 		for b := bbuckets; b != nil; b = b.allnext {
-			bp := (*_4_)(unsafe.Pointer(&b.data))
+			bp := (*bprofrecord)(unsafe.Pointer(&b.data))
 			p[idx].Count = int64(bp.count)
 			p[idx].Cycles = int64(bp.cycles)
 			i := 0

src/pkg/runtime/mprof.h

@@ -13,7 +13,7 @@ struct Bucket
 	// this one is fine because it does not contain pointers.
 	union
 	{
-		struct  // typ == MProf
+		struct MProfRecord // typ == MProf
 		{
 			// The following complex 3-stage scheme of stats accumulation
 			// is required to obtain a consistent picture of mallocs and frees
@@ -43,7 +43,7 @@ struct Bucket
 			uintptr	recent_free_bytes;
 
 		} mp;
-		struct  // typ == BProf
+		struct BProfRecord // typ == BProf
 		{
 			int64	count;
 			int64	cycles;