C++: Make the padding test independent of the predefined_macros file

The padding test is the only test that currently depends on the contents
of the `predefined_macros` flile that we ship with CodeQL for use with the
CodeQL tests. Explicitly specifying `__x86_64` makes the test independent of
the contents of the file.

Author: jketema

cpp/ql/test/library-tests/padding/options

@@ -0,0 +1 @@
+semmle-extractor-options: -D__x86_64=1