[LLVM] Fix runtimes builds not triggering without LLVM_ENABLE_RUNTIMES (llvm#94225)

Summary:
The runtimes builds create separate projects. Normally these use
`LLVM_EANBLE_RUNTIMES`. However, we can also use `LLVM_RUNTIME_TARGETS`
and `RUNTIMES_<target>_LLVM_ENABLE_RUNTIMES` to enable them. Currently,
if `LLVM_RUNTIME_TARGETS` isn't specified this is completely ignored.
This patch fixes the behavior so we can only specify the cross-compiling
versions.

Author: jhuber6

llvm/runtimes/CMakeLists.txt

@@ -430,7 +430,17 @@ function(runtime_register_target name)
   endforeach()
 endfunction()
 
+# Check if we have any runtimes to build.
 if(runtimes)
+  set(build_runtimes TRUE)
+endif()
+foreach(name ${LLVM_RUNTIME_TARGETS})
+  if(RUNTIMES_${name}_LLVM_ENABLE_RUNTIMES)
+    set(build_runtimes TRUE)
+  endif()
+endforeach()
+
+if(build_runtimes)
   # Create a runtimes target that uses this file as its top-level CMake file.
   # The runtimes target is a configuration of all the runtime libraries
   # together in a single CMake invocation.