Rename CACHE_LINE_SIZE to PG_CACHE_LINE_SIZE.

As noted in http://bugs.debian.org/763098 there is a conflict between
postgres' definition of CACHE_LINE_SIZE and the definition by various
*bsd platforms. It's debatable who has the right to define such a
name, but postgres' use was only introduced in 375d8526f290 (9.4), so
it seems like a good idea to rename it.

Discussion: 20140930195756.GC27407@msg.df7cb.de

Per complaint of Christoph Berg in the above email, although he's not
the original bug reporter.

Backpatch to 9.4 where the define was introduced.