[compiler-rt] Create the umbrella check-compiler-rt test target in non-standalone builds too

I don't see why we can't have this target when built as a project alongside
llvm; especially in such build configs, it's useful to have one single target
for running all tests (in standalone builds, check-all does the same too),

Keep the creation of the check-all target only for standalone builds.

Differential Revision: https://reviews.llvm.org/D147646