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
------
REL9_5_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/3e69bf3b142be3f4205435ef3baabcae168c6a14

Modified Files
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src/backend/storage/lmgr/spin.c | 99 +++++++++++++++++++++++++++++++----------
src/test/regress/regress.c | 47 +++++++++++++++++++
2 files changed, 123 insertions(+), 23 deletions(-)