Revert "XFAIL some PGO tests on AIX until the new linker becomes publicly available."

AIX 7.2 TL5 SP4 and AIX 7.3 TL0 SP2 have been released as of now.
The linker in these OS versions recognizes and properly supports
__start_SECNAME and __stop_SECNAME symbols which are needed for PGO.

This reverts commit 93bb2f16.