cmd/cgo: enable test with non-sensible build tag
The build tag on this file is currently unsatisfiable. It was clearly
supposed to be "linux || freebsd || openbsd", but the test doesn't
actually compile on FreeBSD or OpenBSD because they don't define
SYS_gettid. Change the build tag to just "linux".
Change-Id: Ifaffac5438e1b94a8588b5a00435461aa171a6fc
Reviewed-on: https://go-review.googlesource.com/c/go/+/493603
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>