[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@335900 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed