Use proper linker command for drivers on OpenBSD

This fixes the "Cannot load specified object" / setuid_drv.so: undefined symbol '__guard_local' error that occurs on OpenBSD 5.3. It changes the LD_SHARED variable to use gcc rather than ld. A similar issue is documented here: http://erlang.org/pipermail/erlang-questions/2013-May/073837.html. Tested on OpenBSD 5.3 and FreeBSD 9.