[CMake] Respect CMAKE_STRIP and CMAKE_DSYMUTIL on apple platforms

This allows overriding the strip and dsymutil tools, and updates
iOS.cmake to do so. I've also added libtool to iOS.cmake, but it was
already respecting CMAKE_LIBTOOL if set.

llvm-svn: 335900
diff --git a/llvm/cmake/modules/AddLLVM.cmake b/llvm/cmake/modules/AddLLVM.cmake
index 0f3e343..62a2cfc 100644
--- a/llvm/cmake/modules/AddLLVM.cmake
+++ b/llvm/cmake/modules/AddLLVM.cmake
@@ -1557,7 +1557,10 @@
 
   if(NOT LLVM_EXTERNALIZE_DEBUGINFO_SKIP_STRIP)
     if(APPLE)
-      set(strip_command COMMAND xcrun strip -Sxl $<TARGET_FILE:${name}>)
+      if(NOT CMAKE_STRIP)
+        set(CMAKE_STRIP xcrun strip)
+      endif()
+      set(strip_command COMMAND ${CMAKE_STRIP} -Sxl $<TARGET_FILE:${name}>)
     else()
       set(strip_command COMMAND ${CMAKE_STRIP} -gx $<TARGET_FILE:${name}>)
     endif()
@@ -1571,8 +1574,11 @@
       set_property(TARGET ${name} APPEND_STRING PROPERTY
         LINK_FLAGS " -Wl,-object_path_lto,${lto_object}")
     endif()
+    if(NOT CMAKE_DSYMUTIL)
+      set(CMAKE_DSYMUTIL xcrun dsymutil)
+    endif()
     add_custom_command(TARGET ${name} POST_BUILD
-      COMMAND xcrun dsymutil $<TARGET_FILE:${name}>
+      COMMAND ${CMAKE_DSYMUTIL} $<TARGET_FILE:${name}>
       ${strip_command}
       )
   else()