Update mksyscall*.pl to skip generating calls to "use".
Make build tag handling match what's used in the syscall package.

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

Change-Id: I2d2e823fe1846d2110c07eb1bf976852706ecf60
Reviewed-on: https://go-review.googlesource.com/36715
Run-TryBot: Sameer Ajmani <sameer@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>