cmd/internal/sys: fix placement of loong64 definition

In rebasing the patch series up to CL 339015, the branches were messed
up by me, and changes from v3 to v4 of CL 339009 was lost. Fix the
ordering to restore alphabetical order per original review.

Change-Id: I8e57c96e996c4f962cab684a9d305a8dbdeea43b
Reviewed-on: https://go-review.googlesource.com/c/go/+/347731


Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Meng Zhuo <mzh@golangcn.org>