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
8af2565, 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/[email protected]
Reviewed-by: Andres Freund, Noah Misch, Thomas Munro

Author: hlinnaka

Makefile

@@ -13,6 +13,8 @@
 
 # AIX make defaults to building *every* target of the first rule.  Start with
 # a single-target, empty rule to make the other targets non-default.
+# (We don't support AIX anymore, but if someone tries to build on AIX anyway,
+# at least they'll get the instructions to run 'configure' first.)
 all:
 
 all check install installdirs installcheck installcheck-parallel uninstall clean distclean maintainer-clean dist distcheck world check-world install-world installcheck-world:

config/c-compiler.m4

@@ -137,7 +137,7 @@ if test x"$pgac_cv__128bit_int" = xyes ; then
   AC_CACHE_CHECK([for __int128 alignment bug], [pgac_cv__128bit_int_bug],
   [AC_RUN_IFELSE([AC_LANG_PROGRAM([
 /* This must match the corresponding code in c.h: */
-#if defined(__GNUC__) || defined(__SUNPRO_C) || defined(__IBMC__)
+#if defined(__GNUC__) || defined(__SUNPRO_C)
 #define pg_attribute_aligned(a) __attribute__((aligned(a)))
 #elif defined(_MSC_VER)
 #define pg_attribute_aligned(a) __declspec(align(a))