internal/syscall/unix: loong64 use generic syscall

Contributors to the loong64 port are:
  Weining Lu <[email protected]>
  Lei Wang <[email protected]>
  Lingqin Gong <[email protected]>
  Xiaolin Zhao <[email protected]>
  Meidan Li <[email protected]>
  Xiaojuan Zhai <[email protected]>
  Qiyuan Pu <[email protected]>
  Guoqi Chen <[email protected]>

This port has been updated to Go 1.15.6:
  https://github.com/loongson/go

Updates #46229

Change-Id: I5988bf3efed37b03b9193f1089dfece060ccba99
Reviewed-on: https://go-review.googlesource.com/c/go/+/363934
Auto-Submit: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Run-TryBot: David Chase <[email protected]>
Reviewed-by: David Chase <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: XiaodongLoong

src/internal/syscall/unix/at_sysnum_fstatat_linux.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.
 
-//go:build arm64 || riscv64
+//go:build arm64 || loong64 || riscv64
 
 package unix
 

src/internal/syscall/unix/sysnum_linux_generic.go

@@ -2,13 +2,13 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build linux && (arm64 || riscv64)
+//go:build linux && (arm64 || loong64 || riscv64)
 
 package unix
 
 // This file is named "generic" because at a certain point Linux started
 // standardizing on system call numbers across architectures. So far this
-// means only arm64 and riscv64 use the standard numbers.
+// means only arm64 loong64 and riscv64 use the standard numbers.
 
 const (
 	getrandomTrap     uintptr = 278