[compiler-rt] Disable ranlib when using libtool

[smeenai]

Summary

libtool already produces a table of contents, and ranlib just gives
spurious errors because it doesn't understand universal binaries.

[phosek]

LGTM

compiler-rt/cmake/Modules/UseLibtool.cmake

if(NOT LIBTOOL_VERSION VERSION_LESS "862")
       set(LIBTOOL_NO_WARNING_FLAG "-no_warning_for_no_symbols")
     endif()
   endif()
 
   foreach(lang ${languages})
     set(CMAKE_${lang}_CREATE_STATIC_LIBRARY
       "\"${CMAKE_LIBTOOL}\" -static ${LIBTOOL_NO_WARNING_FLAG} -o <TARGET> <LINK_FLAGS> <OBJECTS>")
   endforeach()
+
+  # By default, CMake invokes ranlib on a static library after installing it.
+  # libtool will have produced the table of contents for us already, and ranlib
+  # does not understanding universal binaries, so skip this step. It's important
+  # to set it to empty instead of unsetting it to shadow the cache variable, and
+  # we don't want to unset the cache variable to not affect anything outside
+  # this scope.
+  set(CMAKE_RANLIB "")
 endif()
 
 # If DYLD_LIBRARY_PATH is set we need to set it on archiver commands
 if(DYLD_LIBRARY_PATH)
   set(dyld_envar "DYLD_LIBRARY_PATH=${DYLD_LIBRARY_PATH}")
   foreach(lang ${languages})
     foreach(cmd ${CMAKE_${lang}_CREATE_STATIC_LIBRARY})
       list(APPEND CMAKE_${lang}_CREATE_STATIC_LIBRARY_NEW
            "${dyld_envar} ${cmd}")
     endforeach()
     set(CMAKE_${lang}_CREATE_STATIC_LIBRARY
       ${CMAKE_${lang}_CREATE_STATIC_LIBRARY_NEW})
   endforeach()
 endif()