sim: common: move libcommon.a objects to sources

sim: common: move libcommon.a objects to sources

This simplifies the build logic and avoids an Automake bug where the
common_libcommon_a_OBJECTS variable isn't set in the arch libsim.a
DEPENDENCIES for targets that, alphabetically, come before "common".
We aren't affected by that bug with the current code, but as we move
things out of SIM_ALL_RECURSIVE_DEPS and rely on finer dependencies,
we will trip over it.