[compiler-rt] check-builtins isn't created during all-in-one build

[daltenty]

We don't seem to create the check-builtins target anywhere in the build when you configure an all-in-one build with -DENABLE_LLVM_PROJECTS=clang -DENABLE_LLVM_RUNTIMES=compiler-rt.

We'd expect either check-all via check-runtimes to include these tests but we can see they do not:
https://lab.llvm.org/buildbot/#/builders/95/builds/14342/steps/6/logs/stdio
https://lab.llvm.org/buildbot/#/builders/95/builds/14342/steps/7/logs/stdio

This has the effect that (likely most) of the buildbots aren't running the builtins tests, as that's typically how compiler-rt is configured in the builds these days.

[daltenty]

This is most caused to the odd build structure we due to make sure builtins get's built first (as it's a dependency to build the rest of compiler-rt) when doing the all in one build.

The runtime build doesn't include the target:

cd /home/daltenty/llvm/dev/build/runtimes/runtimes-bins && /opt/freeware/bin/cmake --build /home/daltenty/llvm/dev/build/runtimes/runtimes-bins/ --target check-builtins --config Release
ninja: error: unknown target 'check-builtins'

Which makes sense at the builtins are excluded from the normal runtimes build via setting COMPILER_RT_BUILD_BUILTINS:BOOL=Off which disables the tests here

llvm-project/compiler-rt/test/CMakeLists.txt

Line 69 in 3ea45a6

if(COMPILER_RT_BUILD_BUILTINS)

But the buildins build itself uses a source root equal to compiler-rt/lib/builtins:

 /opt/freeware/bin/cmake --regenerate-during-build -S/home/daltenty/llvm/dev/llvm-project/compiler-rt/lib/builtins -B/home/daltenty/llvm/dev/build/runtimes/builtins-bins

meaning we never walk the test directory in that path either, so neither path creates the test targets.