[libcxx] Fix XFAILs for aligned allocation tests

In r339743, I marked several aligned allocation tests as downright
unsupported on macosx in an attempt to unbreak the build. It turns
out that marking them as unuspported whenever we're on OS X is way
too coarse grained. This commit marks the tests as XFAIL with more
granularity.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@347585 91177308-0d34-0410-b5e6-96231b3b80d8
8 files changed