[libcxx][AIX] Reverting XFAILs for test cases after OS update

These test cases were fixed with AIX 73TL1, and are currently passing on AIX machines with that fix. This fix has also been backported to the 7.2 service line. These were tested on a machine with AIX 7.2 TL 5 SP4 installed.

Differential Revision: https://reviews.llvm.org/D148040