unix: add riscv64 tag to endian_little.go

Update golang/go#27532

Change-Id: I302f7db344e7704000b6e0c976519965b4f6a4ee
GitHub-Last-Rev: a18db686b27b2cf5d8bfc266dd9081fa549f93dc
GitHub-Pull-Request: golang/sys#38
Reviewed-on: https://go-review.googlesource.com/c/sys/+/187637
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/endian_little.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 //
-// +build 386 amd64 amd64p32 arm arm64 ppc64le mipsle mips64le
+// +build 386 amd64 amd64p32 arm arm64 ppc64le mipsle mips64le riscv64
 
 package unix