native: add IsBigEndian bool const

Per golang/go#57237 (comment)

Updates golang/go#57237

endian_big.go

@@ -5,4 +5,10 @@ package native
 
 import "encoding/binary"
 
+// Endian is the encoding/binary.ByteOrder implementation for the
+// current CPU's native byte order.
 var Endian = binary.BigEndian
+
+// IsBigEndian is whether the current CPU's native byte order is big
+// endian.
+const IsBigEndian = true

endian_generic.go

@@ -16,12 +16,16 @@ import (
 
 var Endian binary.ByteOrder
 
+var IsBigEndian bool
+
 func init() {
 	b := uint16(0xff) // one byte
 	if *(*byte)(unsafe.Pointer(&b)) == 0 {
 		Endian = binary.BigEndian
+		IsBigEndian = true
 	} else {
 		Endian = binary.LittleEndian
+		IsBigEndian = false
 	}
 	log.Printf("github.com/josharian/native: unrecognized arch %v (%v), please file an issue", runtime.GOARCH, Endian)
 }

endian_little.go

@@ -5,4 +5,10 @@ package native
 
 import "encoding/binary"
 
+// Endian is the encoding/binary.ByteOrder implementation for the
+// current CPU's native byte order.
 var Endian = binary.LittleEndian
+
+// IsBigEndian is whether the current CPU's native byte order is big
+// endian.
+const IsBigEndian = false

endian_test.go

@@ -1,11 +1,20 @@
 package native_test
 
 import (
+	"encoding/binary"
 	"testing"
 
 	"github.com/josharian/native"
 )
 
 func TestPrintEndianness(t *testing.T) {
 	t.Logf("native endianness is %v", native.Endian)
+
+	var want binary.ByteOrder = binary.BigEndian
+	if !native.IsBigEndian {
+		want = binary.LittleEndian
+	}
+	if native.Endian != want {
+		t.Errorf("IsBigEndian = %v not consistent with native.Endian = %T", native.IsBigEndian, want)
+	}
 }