runtime: fix memhash64 on big endian systems

Fixes #21677.

Change-Id: I869dee5f43df5d87d86922681726297e3024c562
Reviewed-on: https://go-review.googlesource.com/59810
Run-TryBot: Michael Munday <mike.munday@ibm.com>
Reviewed-by: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/hash64.go

@@ -95,7 +95,7 @@ func memhash32(p unsafe.Pointer, seed uintptr) uintptr {
 
 func memhash64(p unsafe.Pointer, seed uintptr) uintptr {
 	h := uint64(seed + 8*hashkey[0])
-	h ^= readUnaligned64(p)
+	h ^= uint64(readUnaligned32(p)) | uint64(readUnaligned32(add(p, 4)))<<32
 	h = rotl_31(h*m1) * m2
 	h ^= h >> 29
 	h *= m3