Re-introduce NetBSD support

Re-introduce NetBSD support

[ History added by Mathieu Desnoyers:

commit bd252a04bbbb ("Use urcu/tls-compat.h")
introduce support for OpenBSD, NetBSD and Darwin by using tls-compat.h
in across urcu. (v0.7.0)

commit 9ba261bd496f ("Android: add a compat layer for 'syscall.h'")
added a syscall compatibility layer for Android. (v0.9.0)

commit b10602e3b143 ("Port: Add Solaris support to urcu/syscall-compat.h")
added Solaris support to urcu/syscall-compat.h (v0.9.0), and doing so
added a:

    #else
    #error "Add platform support to urcu/syscall-compat.h"

preventing build to succeed on unlisted platforms. Before that commit,
unlisted platforms would rely on fallback implementations for
urcu_get_thread_id() (getpid()), futex_async() (busy-wait with poll(2)
delay), and futex_noasync() (pthread_cond(3)).

The choice was made at this point to fail build on OSes that were not
tested against in order to allow consideration of better suited
implementations for urcu_get_thread_id(), futex_async(), and
futex_noasync(), and ensure a minimum level of testing.

commit 80a92637ed8c ("Add FreeBSD, DragonFly to syscall-compat.h")
re-introduce support for FreeBSD and DragonFly (v0.12.0).

commit 020351f306d4 ("Add support for OpenBSD")
re-introduced OpenBSD support implementing urcu_get_thread_id() with
getthrid() (v0.13.4).

]

regtest.out:
============================================================================
Testsuite summary for userspace-rcu 0.15.0
============================================================================
TOTAL: 852
PASS:  852
SKIP:  0
XFAIL: 0
FAIL:  0
XPASS: 0
ERROR: 0
============================================================================
TIME=38:58.39 CPU=597.9% (13605.556u 375.860s) SWAPS=0 (157727+3641680)pf (0i+338o) (1Kc+32Kd)

short_bench.out:
============================================================================
Testsuite summary for userspace-rcu 0.15.0
============================================================================
TOTAL: 852
PASS:  852
SKIP:  0
XFAIL: 0
FAIL:  0
XPASS: 0
ERROR: 0
============================================================================
TIME=1:02:33.91 CPU=716.0% (26438.855u 439.871s) SWAPS=0 (166007+2996145)pf (0i+353o) (0Kc+9Kd)

Signed-off-by: Christos Zoulas <christos@zoulas.com>
Tested-by: Christos Zoulas <christos@zoulas.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I0dbefff0c9f1cbf31741864748ff9b46bd4dd537