internal/syscall/unix: add KernelVersionGE

internal/syscall/unix: add KernelVersionGE

There are a few places in the code which checks that the running kernel
is greater than or equal to x.y. The check takes a few lines and the
checking code is somewhat distracting.

Let's abstract this check into a simple function, KernelVersionGE,
and convert the users accordingly.

Add a test case (I'm not sure it has much value, can be dropped).

Change-Id: I8ec91dcc7452363361f95e46794701c0ae57d956
Reviewed-on: https://go-review.googlesource.com/c/go/+/700796
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>