[llvm] r201921 - Use relative rpath so that the installation and build dirs are relocatable.

[Alexander Potapenko]

> Maybe, you can instead call
> set(CMAKE_BUILD_WITH_INSTALL_RPATH OFF)
> in top-level compiler-rt/CMakeLists.txt (if we don't want to build any
> compiler-rt dynamic runtimes with rpath)?
Good idea indeed.
I'd better do that in compiler-rt/lib/asan, because the compiler-rt
libs may actually need the rpath.