[php-src] PR #23145: TSRM: don't use initial-exec on AIX

Pull Request: https://github.com/php/php-src/pull/23145 Author: NattyNarwhal If building for ZTS, it seems that local-exec can sneak in: ``` PHP Warning: PHP Startup: Unable to load dynamic library 'intl' (tried: /QOpenSys/pkgs/lib/php-8.6/extensions/intl ( 0509-022 Cannot load module /QOpenSys/pkgs/lib/php-8.6/extensions/intl. 0509-026 System error: A file or directory in the path name does not exist.), /QOpenSys/pkgs/lib/php-8.6/extensions/intl.so ( 0509-130 Symbol resolution failed for /QOpenSys/pkgs/lib/php-8.6/extensions/intl.so because: 0509-189 Module /QOpenSys/pkgs/lib/php-8.6/extensions/intl.so is loaded dynamically, but the initial-exec model is used by the module to refer to its own thread-local variables. 0509-191 Examine .loader section relocation entries with the 'dump -Rv' command. 0509-192 Examine .loader section symbols with the 'dump -Tv' command.)) in Unknown on line 0 ``` When inspecting the module for initial-exec: ``` $ dump -Rv /QOpenSys/pkgs/lib/php-8.6/extensions/intl.so | grep TLS_In 0x20014370 0x000001fb TLS_InExc 0x0003 _tsrm_ls_cache ``` It seems it only was in ext/intl/timezone/timezone_class.cpp. However, it seems prudent to just not use local-exec for this case since it seems to be an optimization only really possible on glibc; play it safe elsewhere. (Perhaps this should be really an ifdef GLIBC check?)