[SystemZ][z/OS][libcxx] mark aligned allocation tests UNSUPPORTED on z/OS

zOS doesn't support aligned allocation, so these tests are failing.
For more details on aligned allocation in zOS, see
https://reviews.llvm.org/D87611 and https://reviews.llvm.org/D90178

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D102798