Remove AIX support

There isn't a lot of user demand for AIX support, we have a bunch of
hacks to work around AIX-specific compiler bugs and idiosyncrasies,
and no one has stepped up to the plate to properly maintain it.
Remove support for AIX to get rid of that maintenance overhead. It's
still supported for stable versions.

The acute issue that triggered this decision was that after commit
8af2565248, the AIX buildfarm members have been hitting this
assertion:

    TRAP: failed Assert("(uintptr_t) buffer == TYPEALIGN(PG_IO_ALIGN_SIZE, buffer)"), File: "md.c", Line: 472, PID: 2949728

Apperently the "pg_attribute_aligned(a)" attribute doesn't work on AIX
for values larger than PG_IO_ALIGN_SIZE, for a static const variable.
That could be worked around, but we decided to just drop the AIX support
instead.

Discussion: https://www.postgresql.org/message-id/20240224172345.32@rfd.leadboat.com
Reviewed-by: Andres Freund, Noah Misch, Thomas Munro

src/include/storage/s_lock.h

@@ -414,12 +414,6 @@ typedef unsigned int slock_t;
  * an isync is a sufficient synchronization barrier after a lwarx/stwcx loop.
  * But if the spinlock is in ordinary memory, we can use lwsync instead for
  * better performance.
- *
- * Ordinarily, we'd code the branches here using GNU-style local symbols, that
- * is "1f" referencing "1:" and so on.  But some people run gcc on AIX with
- * IBM's assembler as backend, and IBM's assembler doesn't do local symbols.
- * So hand-code the branch offsets; fortunately, all PPC instructions are
- * exactly 4 bytes each, so it's not too hard to count.
  */
 static __inline__ int
 tas(volatile slock_t *lock)
@@ -430,15 +424,17 @@ tas(volatile slock_t *lock)
 	__asm__ __volatile__(
 "	lwarx   %0,0,%3,1	\n"
 "	cmpwi   %0,0		\n"
-"	bne     $+16		\n"		/* branch to li %1,1 */
+"	bne     1f			\n"
 "	addi    %0,%0,1		\n"
 "	stwcx.  %0,0,%3		\n"
-"	beq     $+12		\n"		/* branch to lwsync */
+"	beq     2f			\n"
+"1: \n"
 "	li      %1,1		\n"
-"	b       $+12		\n"		/* branch to end of asm sequence */
+"	b       3f			\n"
+"2: \n"
 "	lwsync				\n"
 "	li      %1,0		\n"
-
+"3: \n"
 :	"=&b"(_t), "=r"(_res), "+m"(*lock)
 :	"r"(lock)
 :	"memory", "cc");
@@ -666,21 +662,6 @@ tas(volatile slock_t *lock)
 
 #if !defined(HAS_TEST_AND_SET)	/* We didn't trigger above, let's try here */
 
-#if defined(_AIX)	/* AIX */
-/*
- * AIX (POWER)
- */
-#define HAS_TEST_AND_SET
-
-#include <sys/atomic_op.h>
-
-typedef int slock_t;
-
-#define TAS(lock)			_check_lock((slock_t *) (lock), 0, 1)
-#define S_UNLOCK(lock)		_clear_lock((slock_t *) (lock), 0)
-#endif	 /* _AIX */
-
-
 /* These are in sunstudio_(sparc|x86).s */
 
 #if defined(__SUNPRO_C) && (defined(__i386) || defined(__x86_64__) || defined(__sparc__) || defined(__sparc))