elf-core: Run-time reg context selection

Instantiate RegisterContextCore... based on getOS() instead of with
compile-time #ifdef-ery.

The assert()s here are unfortunate, but better than crashing with no
explanation.  (This would previously happen for an unsupported
architecture, anyhow.)

We should add an equivalent OS and architecture test to
ProcessElfCore::DoLoadCore() and cleanly report the error to the user.