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.
0556a2ba53