Finishes cleanups that I started in other commits in this branch.

 * Use common.linkage for all exports instead of redoing the logic in
   each file.
 * Remove pointless `@setRuntimeSafety` calls.
 * Avoid redundantly exporting multiple versions of functions. For
   example, if PPC wants `ceilf128` then don't also export `ceilq`;
   similarly if ARM wants `__aeabi_ddiv` then don't also export
   `__divdf3`.
 * Use `inline` for helper functions instead of making inline calls at
   callsites.