The SyscallNoError and RawSyscallNoError functions are only needed on
Linux. Make sure they are not used within the generated zsyscall_*.go
files if $GOOS != "linux".

Updates golang/go#22924

Change-Id: I3b2aa8624895b0527dcc832b2945a8d591f0ba1a
Reviewed-on: https://go-review.googlesource.com/90475
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>