cpu: define IsBigEndian on wasm

For golang/go#57237

Change-Id: Ia2aa943e93c8df51cd08003535fa026d2bb8003e
Reviewed-on: https://go-review.googlesource.com/c/sys/+/494856
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>

cpu/endian_little.go

@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build 386 || amd64 || amd64p32 || alpha || arm || arm64 || loong64 || mipsle || mips64le || mips64p32le || nios2 || ppc64le || riscv || riscv64 || sh
-// +build 386 amd64 amd64p32 alpha arm arm64 loong64 mipsle mips64le mips64p32le nios2 ppc64le riscv riscv64 sh
+//go:build 386 || amd64 || amd64p32 || alpha || arm || arm64 || loong64 || mipsle || mips64le || mips64p32le || nios2 || ppc64le || riscv || riscv64 || sh || wasm
+// +build 386 amd64 amd64p32 alpha arm arm64 loong64 mipsle mips64le mips64p32le nios2 ppc64le riscv riscv64 sh wasm
 
 package cpu