[runtimes] Use a response file for runtimes test suites

We don't know which test suites are going to be included by runtimes
builds so we cannot include those before running the sub-build, but
that's not possible during the LLVM build configuration. We instead use
a response file that's populated by the runtimes build as a level of
indirection.

This addresses the issue described in:
https://discourse.llvm.org/t/cmake-regeneration-is-broken/62788

Differential Revision: https://reviews.llvm.org/D132438