This makes dynarmic installable, and also adds a CMake package config
file, that allows projects to use `find_package(dynarmic)` to import the
library.
I know #636 adds the same thing, but while experimenting with the
different install options in
https://github.com/merryhime/dynarmic/pull/636#discussion_r725656034
I ended up with a working patch, so I'm proposing this as well. This
implements solution 2.
This gets rid of the need to store to individual variables before creating
the target itself, cleaning up the variables in the surrounding scope a little bit.
Moves functions out of the main CMakeLists file into module files that
can just be included whenever necessary. This also uses the CMake
provided variables for enforcing compiler requirements.
