[libc++][C++03] Don't run cstdalign.compile.pass.cpp (#163357)

`__alignas_is_defined` and `__alignof_is_defined` are a C++11 feature
which we only recently added. I don't think it will break anybody if we
don't provide these macros in C++03, so this simply disable the test
instead.

Author: philnik777

libcxx/test/std/language.support/support.runtime/cstdalign.compile.pass.cpp

@@ -10,7 +10,7 @@
 
 // ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS
 
-// XFAIL: FROZEN-CXX03-HEADERS-FIXME
+// UNSUPPORTED: c++03
 
 #include <cstdalign>