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/interfaces/libpq/Makefile

@@ -114,7 +114,7 @@ backend_src = $(top_srcdir)/src/backend
 # coding rule.
 libpq-refs-stamp: $(shlib)
 ifneq ($(enable_coverage), yes)
-ifeq (,$(filter aix solaris,$(PORTNAME)))
+ifeq (,$(filter solaris,$(PORTNAME)))
 	@if nm -A -u $< 2>/dev/null | grep -v -e __cxa_atexit -e __tsan_func_exit | grep exit; then \
 		echo 'libpq must not be calling any function which invokes exit'; exit 1; \
 	fi

src/interfaces/libpq/meson.build

@@ -54,9 +54,8 @@ libpq_c_args = ['-DSO_MAJOR_VERSION=5']
 #    libpq_st, and {pgport,common}_shlib for libpq_sh
 #
 # We could try to avoid building the source files twice, but it probably adds
-# more complexity than its worth (AIX doesn't support link_whole yet, reusing
-# object files requires also linking to the library on windows or breaks
-# precompiled headers).
+# more complexity than its worth (reusing object files requires also linking
+# to the library on windows or breaks precompiled headers).
 libpq_st = static_library('libpq',
   libpq_sources,
   include_directories: [libpq_inc],