cpu: remove encoding/binary dependency

It's unnecessarily heavy, bringing in reflect, etc.

I'd like for this package to become a leaf package with no
dependencies so we can use it from the Go runtime directly and remove
internal/cpu.

Change-Id: I1794854add9ad87cc13b685bae4a039e1449c22d
Reviewed-on: https://go-review.googlesource.com/c/sys/+/194646
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Martin Möhrmann <moehrmann@google.com>

cpu/byteorder.go

@@ -5,26 +5,56 @@
 package cpu
 
 import (
-	"encoding/binary"
 	"runtime"
 )
 
+// byteOrder is a subset of encoding/binary.ByteOrder.
+type byteOrder interface {
+	Uint32([]byte) uint32
+	Uint64([]byte) uint64
+}
+
+type littleEndian struct{}
+type bigEndian struct{}
+
+func (littleEndian) Uint32(b []byte) uint32 {
+	_ = b[3] // bounds check hint to compiler; see golang.org/issue/14808
+	return uint32(b[0]) | uint32(b[1])<<8 | uint32(b[2])<<16 | uint32(b[3])<<24
+}
+
+func (littleEndian) Uint64(b []byte) uint64 {
+	_ = b[7] // bounds check hint to compiler; see golang.org/issue/14808
+	return uint64(b[0]) | uint64(b[1])<<8 | uint64(b[2])<<16 | uint64(b[3])<<24 |
+		uint64(b[4])<<32 | uint64(b[5])<<40 | uint64(b[6])<<48 | uint64(b[7])<<56
+}
+
+func (bigEndian) Uint32(b []byte) uint32 {
+	_ = b[3] // bounds check hint to compiler; see golang.org/issue/14808
+	return uint32(b[3]) | uint32(b[2])<<8 | uint32(b[1])<<16 | uint32(b[0])<<24
+}
+
+func (bigEndian) Uint64(b []byte) uint64 {
+	_ = b[7] // bounds check hint to compiler; see golang.org/issue/14808
+	return uint64(b[7]) | uint64(b[6])<<8 | uint64(b[5])<<16 | uint64(b[4])<<24 |
+		uint64(b[3])<<32 | uint64(b[2])<<40 | uint64(b[1])<<48 | uint64(b[0])<<56
+}
+
 // hostByteOrder returns binary.LittleEndian on little-endian machines and
 // binary.BigEndian on big-endian machines.
-func hostByteOrder() binary.ByteOrder {
+func hostByteOrder() byteOrder {
 	switch runtime.GOARCH {
 	case "386", "amd64", "amd64p32",
 		"arm", "arm64",
 		"mipsle", "mips64le", "mips64p32le",
 		"ppc64le",
 		"riscv", "riscv64":
-		return binary.LittleEndian
+		return littleEndian{}
 	case "armbe", "arm64be",
 		"mips", "mips64", "mips64p32",
 		"ppc", "ppc64",
 		"s390", "s390x",
 		"sparc", "sparc64":
-		return binary.BigEndian
+		return bigEndian{}
 	}
 	panic("unknown architecture")
 }