runtime: use first line of cpuset output on FreeBSD

runtime: use first line of cpuset output on FreeBSD

Fix TestFreeBSDNumCPU on newer versions of FreeBSD which have multi line
output from cpuset e.g.

cpuset -g -p 4141
pid 4141 mask: 0, 1, 2, 3, 4, 5, 6, 7, 8
pid 4141 domain policy: first-touch mask: 0, 1

The test now uses just the first line of output.

Fixes #38937
Fixes #25924

Change-Id: If082ee6b82120ebde4dc437e58343b3dad69c65f
Reviewed-on: https://go-review.googlesource.com/c/go/+/232801
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>