Move R2R-specific code to be outside FEATURE_PREJIT (#24075)

Move R2R-specific code to be outside FEATURE_PREJIT (#24075)

This refactoring is preparation for disabling fragile NGen support in the runtime. It keeps fragile-NGen specific code under FEATURE_PREJIT and moves the code required to support R2R to be outside FEATURE_PREJIT.

The eventual goal is to compile the runtime without FEATURE_PREJIT defined to avoid fragile-NGen specific overhead.