Matches other architectures by using names for syscalls instead of
numbers directly.

Fixes #20499.

Change-Id: I63d606b0b1fe6fb517fd994a7542a3f38d80dd54
Reviewed-on: https://go-review.googlesource.com/44213
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>