pgsql: Better express platform requirements in s_lock.h.

Better express platform requirements in s_lock.h.

Presently, s_lock.h insists that every platform provide TAS(), and
its header comment doesn't say which of the macros a new platform
actually needs to supply. The real contract is that a platform
must provide either S_LOCK() or a TAS() for the default S_LOCK() to
be built on.

To better express that, this commit moves the "no spinlock support"
error to where the default S_LOCK() is defined, compiles s_lock()
only when the default S_LOCK() is in use, and provides a default
TAS_SPIN() only when there is a TAS() to base it on. The direct
callers of s_lock() in the spinlock tests are adjusted to match.
The header comment is reworked accordingly, dropping the note about
pre-9.5 volatile requirements and the caution about TAS()
spuriously failing along the way.

Co-authored-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/afkUeI7UhacZ5ZFm%40nathan

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/d9b5a63f49d9d372a6609243eb0e9417a0a35e6f

Modified Files
--------------
src/backend/storage/lmgr/s_lock.c | 4 +++-
src/include/storage/s_lock.h | 46 +++++++++++++++++++--------------------
src/test/regress/regress.c | 2 +-
3 files changed, 26 insertions(+), 26 deletions(-)