[CMake][runtimes] Skip adding 2nd set of the same variables for a generic target

[krisb]

Summary

No need to parse and add the same variables twice if runtimes is being
built for a generic target (i.e. w/o multilib).

[phosek]

LGTM

llvm/runtimes/CMakeLists.txt

function(runtime_register_target name target)
     foreach(variableName ${variableNames})
       string(FIND "${variableName}" "RUNTIMES_${target}_" out)
       if("${out}" EQUAL 0)
         string(REPLACE "RUNTIMES_${target}_" "" new_name ${variableName})
         string(REPLACE ";" "|" new_value "${${variableName}}")
         list(APPEND ${name}_extra_args "-D${new_name}=${new_value}")
       endif()
     endforeach()
+    if(NOT "${name}" STREQUAL "${target}")
       foreach(variableName ${variableNames})
         string(FIND "${variableName}" "RUNTIMES_${name}_" out)
         if("${out}" EQUAL 0)
           string(REPLACE "RUNTIMES_${name}_" "" new_name ${variableName})
           string(REPLACE ";" "|" new_value "${${variableName}}")
           list(APPEND ${name}_extra_args "-D${new_name}=${new_value}")
         endif()
       endforeach()
+    endif()
 
     if(NOT RUNTIMES_${name}_LLVM_ENABLE_RUNTIMES AND NOT RUNTIMES_${target}_LLVM_ENABLE_RUNTIMES)
       string(REPLACE ";" "|" LLVM_ENABLE_RUNTIMES_PASSTHROUGH "${LLVM_ENABLE_RUNTIMES}")
       list(APPEND ${name}_extra_args -DLLVM_ENABLE_RUNTIMES=${LLVM_ENABLE_RUNTIMES_PASSTHROUGH})
     endif()
 
     if(target IN_LIST LLVM_RUNTIME_BUILD_ID_LINK_TARGETS)
       list(APPEND EXTRA_ARGS STRIP_TOOL ${CMAKE_CURRENT_BINARY_DIR}/llvm-strip-link)