[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

(cherry picked from commit f640c31)

libcxx/test/libcxx/atomics/atomics.align/align.pass.cpp

@@ -20,6 +20,12 @@
 // std::atomic when used with __attribute__((vector(X))).
 // XFAIL: gcc
 
+// This fails on PowerPC, as the LLIArr2 and Padding structs do not have
+// adequate alignment, despite these types returning true for the query of
+// being lock-free. This is an issue that occurs when linking in the
+// PowerPC GNU libatomic library into the test.
+// XFAIL: target=powerpc{{.*}}le-unknown-linux-gnu
+
 // <atomic>
 
 // Verify that the content of atomic<T> is properly aligned if the type is