aix: revert TLS common change

GCC uses TLS common for both public common / BSS and local common / BSS.
This patch reverts to use .comm directive to allocate TLS
common / BSS.  This also changes the priority of section selection
to use BSS before data section.

gcc/ChangeLog:

	* config/rs6000/rs6000.c (rs6000_xcoff_select_section): Select
	TLS BSS before TLS data.
	* config/rs6000/xcoff.h (ASM_OUTPUT_TLS_COMMON): Use .comm.

gcc/testsuite/ChangeLog:

	* g++.dg/gomp/tls-5.C: Expect tbss failure on AIX.