[CMake] Create generic runtimes targets as needed (#76096)

We currently create a generic runtime target for all subbuilds. For
example if we're building libcxx for aarch64-linux-gnu and
x86_64-linux-gnu, we would create the cxx target that would depend on
cxx-aarch64-linux-gnu and cxx-x86_64-linux-gnu. The current
implementation creates the generic runtimes targets ahead of time which
introduces an issue where different subbuilds enable different runtimes.
We should instead create the generic targets as needed.

Author: petrhosek

llvm/runtimes/CMakeLists.txt

@@ -391,8 +391,17 @@ function(runtime_register_target name)
     add_dependencies(runtimes-test-depends runtimes-test-depends-${name})
   endif()
   foreach(runtime_name ${runtime_names})
+    if(NOT TARGET ${runtime_name})
+      add_custom_target(${runtime_name})
+    endif()
     add_dependencies(${runtime_name} ${runtime_name}-${name})
+    if(NOT TARGET install-${runtime_name})
+      add_custom_target(install-${runtime_name})
+    endif()
     add_dependencies(install-${runtime_name} install-${runtime_name}-${name})
+    if(NOT TARGET install-${runtime_name}-stripped)
+      add_custom_target(install-${runtime_name}-stripped)
+    endif()
     add_dependencies(install-${runtime_name}-stripped install-${runtime_name}-${name}-stripped)
   endforeach()
   foreach(component ${LLVM_RUNTIME_DISTRIBUTION_COMPONENTS})
@@ -459,11 +468,6 @@ if(runtimes)
         add_custom_target(runtimes-test-depends)
         set(test_targets "")
       endif()
-      foreach(runtime_name ${RUNTIME_NAMES})
-        add_custom_target(${runtime_name})
-        add_custom_target(install-${runtime_name})
-        add_custom_target(install-${runtime_name}-stripped)
-      endforeach()
       if(LLVM_RUNTIME_DISTRIBUTION_COMPONENTS)
         foreach(component ${LLVM_RUNTIME_DISTRIBUTION_COMPONENTS})
           add_custom_target(${component})