[compiler-rt/cmake] Don't attempt to ad-hoc sign for OS X

[loladiro]

Summary

Revision rL301443 added an ad-hoc codesigning step when building compiler-rt
dylibs. My understanding is that this is needed for testing such dylibs in
the iOS simulator. However, adding such a step is problematic for cross
compilation to OS X from other platforms, as the codesign utility is
generally not available in a cross compilation environment. Since there is
AFAIK no benefit to ad-hoc signing these libraries on OS X, restrict the
extra codesign step to IOS.

compiler-rt/cmake/Modules/AddCompilerRT.cmake

foreach(libname ${libnames})
     if(${type} STREQUAL "SHARED")
       if(COMMAND llvm_setup_rpath)
         llvm_setup_rpath(${libname})
       endif()
       if(WIN32 AND NOT CYGWIN AND NOT MINGW)
         set_target_properties(${libname} PROPERTIES IMPORT_PREFIX "")
         set_target_properties(${libname} PROPERTIES IMPORT_SUFFIX ".lib")
       endif()
-      if(APPLE)
+      if(IOS)
         # Ad-hoc sign the dylibs
         add_custom_command(TARGET ${libname}
           POST_BUILD  
           COMMAND codesign --sign - $<TARGET_FILE:${libname}>
           WORKING_DIRECTORY ${COMPILER_RT_LIBRARY_OUTPUT_DIR}
         )
       endif()
     endif()