Recognize NetBSD in:
 - go/internal/work/init.go
 - race.bash
 - runtime/race/race.go

Add __ps_strings symbol in runtime/cgo/netbsd.go as this is
used internally in the TSan library for NetBSD and used for
ReExec().

Tested on NetBSD/amd64 v. 8.99.12.

Around 98% tests are passing for the ./race.bash target.

Updates #19273

Change-Id: Ic0e48d2fb159a7868aab5e17156eeaca1225e513
GitHub-Last-Rev: d6e082707b9b18df1fe63f723666f4d2eb5e6cfe
GitHub-Pull-Request: golang/go#24322
Reviewed-on: https://go-review.googlesource.com/99835Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>