Issue Description:

• swift/utils/build-script-impl uses xcrun --find to find the lipo it uses to create static libraries

• swift/CMakeLists.txt uses find_program() to find the lipo it uses to make universal binaries

These could return two different lipo executables, especially if the person building has specified a custom --darwin-xcrun-toolchain when invoking swift/utils/build-script.

Instead, build-script-impl should be modified such that it passes the path to lipo to CMake.