[runtimes] Set more paths when building runtimes standalone

These paths are needed when building with per-target runtime directories.

(It's possible to fix this by manually setting these when invoking
cmake, but one isn't supposed to need to do that.)

Also set LLVM_TOOLS_BINARY_DIR while touching this area (as it's
also unset in this case) even if it isn't specifically needed by the
per-target runtime configuration.

Differential Revision: https://reviews.llvm.org/D107895