[CMake] Pass CMAKE_INSTALL_DO_STRIP to external projects

This is necessary to make install-<target>-stripped work for
external projects such as runtimes.

Differential Revision: https://reviews.llvm.org/D49335

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@337115 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed