cpu: add linux/arm64

Port from Go internal/cpu

Updates golang/go#25185

Change-Id: I8390980e38b61f6c428fafa0665a03952e7b00bb
Reviewed-on: https://go-review.googlesource.com/c/150718
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Martin Möhrmann <moehrmann@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

cpu/cpu_test.go

@@ -26,3 +26,15 @@ func TestAVX2hasAVX(t *testing.T) {
 		}
 	}
 }
+
+func TestARM64minimalFeatures(t *testing.T) {
+	if runtime.GOARCH != "arm64" || runtime.GOOS != "linux" {
+		return
+	}
+	if !cpu.ARM64.HasASIMD {
+		t.Fatal("HasASIMD expected true, got false")
+	}
+	if !cpu.ARM64.HasFP {
+		t.Fatal("HasFP expected true, got false")
+	}
+}