Remove spinlock support for SINIX, Sun3, and NS32K.

All of these platforms are very much obsolete.

As far as I can determine, the last version of SINIX, later renamed
Reliant, occurred some time between 2002 and 2005.

The last release of SunOS that would run on a sun3 was released in
November of 1991; the last release of OpenBSD which supported that
platform was in 2001.  The highest clock speed of any processor in
the family was 25MHz.

The NS32K (national semiconductor 320xx) architecture was retired
in 1990.

Support can be re-added if a maintainer emerges for any of these
platforms, but it seems unlikely.

Reviewed by Andres Freund.