pgsql: Fix deadlock danger when atomic ops are done under spinlock.

Fix deadlock danger when atomic ops are done under spinlock.

This was a danger only for --disable-spinlocks in combination with
atomic operations unsupported by the current platform.

While atomics.c was careful to signal that a separate semaphore ought
to be used when spinlock emulation is active, spin.c didn't actually
implement that mechanism. That's my (Andres') fault, it seems to have
gotten lost during the development of the atomic operations support.

Fix that issue and add test for nesting atomic operations inside a
spinlock.

Author: Andres Freund
Discussion: https://postgr.es/m/20200605023302.g6v3ydozy5txifji@alap3.anarazel.de
Backpatch: 9.5-

Branch
------
REL_12_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/6cc2866c4cdf6a167be1ed9cb0fff9fce55758d6

Modified Files
--------------
src/backend/storage/lmgr/spin.c | 97 ++++++++++++++++++++++++++++-------------
src/test/regress/regress.c      | 52 ++++++++++++++++++++++
2 files changed, 119 insertions(+), 30 deletions(-)