[CMake][runtimes] Add install target for runtimes builtins

[phosek]

Summary

This adds an install-builtins target to avoid having to list all builtins targets explicitly.

[beanz]

LGTM!

runtimes/CMakeLists.txt

if(NOT LLVM_BUILTIN_TARGETS)
                                           -DCMAKE_C_COMPILER_TARGET=${TARGET_TRIPLE}
                                           -DCMAKE_ASM_COMPILER_TARGET=${TARGET_TRIPLE}
                                PASSTHROUGH_PREFIXES COMPILER_RT
                                USE_TOOLCHAIN
                                ${EXTRA_ARGS})
     else()
       get_cmake_property(variableNames VARIABLES)
       add_custom_target(builtins)
+      add_custom_target(install-builtins)
       foreach(target ${LLVM_BUILTIN_TARGETS})
         string(REPLACE "-" ";" builtin_target_list ${target})
         foreach(item ${builtin_target_list})
           string(TOLOWER "${item}" item_lower)
           if(item_lower MATCHES "darwin")
             message(FATAL_ERROR "LLVM_BUILTIN_TARGETS isn't implemented for Darwin platform!")
           endif()
         endforeach()
else()
                                           -DCMAKE_ASM_COMPILER_WORKS=On
                                           -DCOMPILER_RT_DEFAULT_TARGET_ONLY=On
                                           ${${target}_extra_args}
                                TOOLCHAIN_TOOLS clang lld llvm-ar llvm-ranlib
                                PASSTHROUGH_PREFIXES COMPILER_RT
                                USE_TOOLCHAIN
                                ${EXTRA_ARGS})
         add_dependencies(builtins builtins-${target})
+        add_dependencies(install-builtins install-builtins-${target})
       endforeach()
     endif()
     set(deps builtins)
     # We don't need to depend on the builtins if we're building instrumented
     # because the next stage will use the same compiler used to build this stage.
     if(NOT LLVM_BUILD_INSTRUMENTED AND CLANG_ENABLE_BOOTSTRAP)
       add_dependencies(clang-bootstrap-deps builtins)
     endif()