Update mksyscall.pl to skip generating calls to "use".

See issue https://golang.org/issue/16607
and CL https://golang.org/cl/36616

Change-Id: Id36614fce5d98a8538edf4f4a7276e650b99b839
Reviewed-on: https://go-review.googlesource.com/c/156437Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>