recommended way of making changes and updating archive:
rm -rf foo
+mkdir foo
cd foo
ar x ../cpu-os-macros.a
<perform your changes>
cd -
-ar cDr cpu-os-macros.a foo/*macros
+# preserve order of files within the archive
+LC_ALL=C /bin/sh -c 'ar cDr cpu-os-macros.a foo/*macros'