syscall, runtime/internal/syscall: always zero the higher bits of return value on...

syscall, runtime/internal/syscall: always zero the higher bits of return value on linux/loong64

All loong64 syscalls return values only via R4/A0, and R5/A1 may contain unrelated
content. Always zero the second return value.

Change-Id: I62af59369bece5bd8028b937c74f4694150f7a55
Reviewed-on: https://go-review.googlesource.com/c/go/+/411615
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Austin Clements <austin@google.com>