hash/maphash: move purego-only helper functions to build tagged file

Hash.float64 and btoi helper functions are used only in the purego
version. Move them to the build tagged file.

Change-Id: I57f9a48966573ab0aee1de759eeddd2331967870
Reviewed-on: https://go-review.googlesource.com/c/go/+/675158
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/hash/maphash/maphash.go

@@ -15,8 +15,6 @@ package maphash
 import (
 	"hash"
 	"internal/abi"
-	"internal/byteorder"
-	"math"
 )
 
 // A Seed is a random value that selects the specific hash function
@@ -310,25 +308,3 @@ func WriteComparable[T comparable](h *Hash, x T) {
 	}
 	writeComparable(h, x)
 }
-
-func (h *Hash) float64(f float64) {
-	if f == 0 {
-		h.WriteByte(0)
-		return
-	}
-	var buf [8]byte
-	if f != f {
-		byteorder.LEPutUint64(buf[:], randUint64())
-		h.Write(buf[:])
-		return
-	}
-	byteorder.LEPutUint64(buf[:], math.Float64bits(f))
-	h.Write(buf[:])
-}
-
-func btoi(b bool) byte {
-	if b {
-		return 1
-	}
-	return 0
-}

src/hash/maphash/maphash_purego.go

@@ -10,6 +10,7 @@ import (
 	"crypto/rand"
 	"errors"
 	"internal/byteorder"
+	"math"
 	"math/bits"
 	"reflect"
 )
@@ -175,3 +176,25 @@ func appendT(h *Hash, v reflect.Value) {
 	}
 	panic(errors.New("maphash: hash of unhashable type " + v.Type().String()))
 }
+
+func (h *Hash) float64(f float64) {
+	if f == 0 {
+		h.WriteByte(0)
+		return
+	}
+	var buf [8]byte
+	if f != f {
+		byteorder.LEPutUint64(buf[:], randUint64())
+		h.Write(buf[:])
+		return
+	}
+	byteorder.LEPutUint64(buf[:], math.Float64bits(f))
+	h.Write(buf[:])
+}
+
+func btoi(b bool) byte {
+	if b {
+		return 1
+	}
+	return 0
+}