tags: honor COMPILED_SOURCE with apart output directory

tags: honor COMPILED_SOURCE with apart output directory

When the kernel is compiled with an "O=" argument, the object files are
not in the source tree, but in the build tree.

This patch fixes O= build by looking for object files in the build tree.

Fixes: 923e02ecf3f8 ("scripts/tags.sh: Support compiled source")
Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>