aix: 64 bit AIX TLS libpthread dependency.

aix: 64 bit AIX TLS libpthread dependency.

64bit XCOFF files will generated TLS access, with local-exec or
global-exec models, by an access to R13. Thus, there isn't
any reference to a TLS symbol.

The problem is that it allows programs with TLS to be compiled and
linked even without -pthread. Most of the time, it will result in
a segfault when trying to access a TLS variable. But sometimes, it
might create a memory corruption.

This patch forces a reference to __tls_get_addr() to ensure link
will fail without -pthread.

gcc/ChangeLog:
2021-08-11  Clément Chigot  <clement.chigot@atos.net>

	* config/rs6000/rs6000.c (xcoff_tls_exec_model_detected): New.
	(rs6000_legitimize_tls_address_aix): Use it.
	(rs6000_xcoff_file_end): Add ".ref __tls_get_addr" when
	xcoff_tls_exec_model_detected is true.