Back-port Postgres 7.4 spinlock code into 7.3 branch. This adds

Back-port Postgres 7.4 spinlock code into 7.3 branch.  This adds
previously-missing spinlock code for x86_64 and ppc64 architectures,
converts the ppc/ppc64 code into gcc inlines, and provides a better
spinlock backoff algorithm on all architectures.  Aside from being
almost identical to the community 7.4 source code, this exact patch
has been in use for awhile in Red Hat's RHEL3 RPMs, so I have pretty
good confidence in it.  Why bother, you ask?  I'm taking pity on a
couple of buildfarm members that have been vainly trying to build 7.3
on these 64-bit architectures.