[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.

llvm-svn: 347585