runtime.GOOS and runtime.GOARCH are constants, thus all comparisons are
evaluated at compile time.

Follow-up for CL 153619 to make the code consistent with the syscall
package, cf. CL 153837

Change-Id: I0520d88c1636c6ed2acad69fce85a28042eafe56
Reviewed-on: https://go-review.googlesource.com/c/153937
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Joel Sing <joel@sing.id.au>
TryBot-Result: Gobot Gobot <gobot@golang.org>