[runtimes] Fix check-runtimes when target-specific compiler-rt tests are disabled

[tstellar]

It's possible to pass options like -DRUNTIMES_aarch64-linux-android21_LLVM_INCLUDE_TESTS=OFF to cmake which will disable tests for specific compiler-rt targets. llvm/runtimes/CMakeLists.txt assumes that when LLVM_INCLUDE_TESTS is enabled that each target's compiler-rt tests will run too, but this is not the case when the option above is used.

With this patch, the check-runtimes target will do nothing when these options are used instead of emitting an error. It also fixes the test-depends target with this config.

This fix should allow 7f215b1 to be re-committed.

[petrhosek]

The alternative approach I had in mind is definining an empty runtimes-test-depends target in the runtimes build when LLVM_INCLUDE_TESTS is disabled similarly to what we do for the response file:

llvm-project/runtimes/CMakeLists.txt

Lines 250 to 253 in 1459c62

	else()
	# Create empty files so the parent build can use these unconditionally.
	file(TOUCH ${CMAKE_CURRENT_BINARY_DIR}/lit.tests)
	endif()

I implemented that approach in #73629.

[tstellar]

Closing in favor of #73629