Fix linker set creation for GCC 3.4 with -funit-at-a-time.

Fix linker set creation for GCC 3.4 with -funit-at-a-time.

GCC 3.4 optimizes away global statics, which are referenced. Define a new
macro __used, which expands to attribute((used)) and tells the compiler that
a certain static indead referenced. For older versions and other compilers
it defaults to the meaning of __unused to avoid warnings.