[libc++] XFAIL align.pass.cpp for PowerPC LE

This patch XFAILs the `align.pass.cpp` for PowerPC (LE).

It appears that this test will fail on Power for the `LLIArr2` and `Padding` structs within the test,
as the `assert` for `alignof(AtomicImpl) >= sizeof(AtomicImpl)` will be false. In this case, these structs
presumably should not be lock-free, so we currently XFAIL this for now.

The failure was discovered after D97913 was committed. It looks like `alignof(AtomicImpl) < sizeof(AtomicImpl)`,
even prior to this commit, but this test began running on Power after D97913, whereas we were
not running `align.pass.cpp` before.

This patch addresses https://bugs.llvm.org/show_bug.cgi?id=51548 by temporarily XFAILing the test
in order to investigate it further.

Differential Revision: https://reviews.llvm.org/D108668