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.
