runtime: add Benchmark[Clear|Copy]Fat[8|12]

These correspond to 2 and 3 word fat copies/clears on 8g, which dominate usage in the stdlib. (70% of copies and 46% of clears are for 2 or 3 words.) I missed these in CL 111350043, which added 2 and 3 word benchmarks for 6g. A follow-up CL will optimize these cases.

LGTM=khr
R=khr
CC=golang-codereviews
https://golang.org/cl/115160043

src/pkg/runtime/memmove_test.go

@@ -162,6 +162,18 @@ func BenchmarkMemclr256(b *testing.B)   { bmMemclr(b, 256) }
 func BenchmarkMemclr4096(b *testing.B)  { bmMemclr(b, 4096) }
 func BenchmarkMemclr65536(b *testing.B) { bmMemclr(b, 65536) }
 
+func BenchmarkClearFat8(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		var x [8 / 4]uint32
+		_ = x
+	}
+}
+func BenchmarkClearFat12(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		var x [12 / 4]uint32
+		_ = x
+	}
+}
 func BenchmarkClearFat16(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		var x [16 / 4]uint32
@@ -211,6 +223,20 @@ func BenchmarkClearFat1024(b *testing.B) {
 	}
 }
 
+func BenchmarkCopyFat8(b *testing.B) {
+	var x [8 / 4]uint32
+	for i := 0; i < b.N; i++ {
+		y := x
+		_ = y
+	}
+}
+func BenchmarkCopyFat12(b *testing.B) {
+	var x [12 / 4]uint32
+	for i := 0; i < b.N; i++ {
+		y := x
+		_ = y
+	}
+}
 func BenchmarkCopyFat16(b *testing.B) {
 	var x [16 / 4]uint32
 	for i := 0; i < b.N; i++ {