CL 470298 defaulted to external linking for Go programs that import the standard-library packages net, os/user, and crypto/x509 when cgo is used to build those packages.

The tests for golang.org/x/sys/unix happen to import the net package:
https://cs.opensource.google/search?q=%5C%22net%5C%22&ss=go%2Fx%2Fsys:unix%2F

Somehow, the change in linking seems to have caused all of the netbsd builders to start failing unix.TestMmap (https://build.golang.org/log/1335867d48cb1aa20e21d276efe7e282e1f4938e):

--- FAIL: TestMmap (0.00s)
    mmap_unix_test.go:23: Mprotect: permission denied
FAIL
FAIL	golang.org/x/sys/unix	0.476s

It isn't clear to me why changing the linker behavior would have that effect, but there is a clear inflection point in the dashboard logs at that CL (https://build.golang.org/?repo=golang.org%2fx%2fsys).

(attn @golang/compiler @golang/netbsd)