[runtimes] Add the ability to include out of tree runtimes

[ldionne]

Summary

LLVM_EXTERNAL_RUNTIMES is similar to LLVM_EXTERNAL_PROJECTS: the
source directory for the external runtime is resolved through the
LLVM_EXTERNAL_<proj>_SOURCE_DIR variable.

This makes it easier to experiment with additional runtimes locally
and provides a relatively low maintenance hook for downstreams to
add their own runtimes outside of the LLVM tree.

[Mordante]

llvm/docs/CMake.rst

634

IMO There should be a way to pass additional arguments to CMake for the external libraries, including documentation how to do that.