[CMake] Avoid passing -rtlib=compiler-rt when using compiler-rt

We build libc++ and libc++abi with -nodefaultlibs, so -rtlib=compiler-rt
has no effect and results in an 'argument unused during compilation'
warning which breaks the build when using -Werror. We can therefore drop
-rtlib=compiler-rt without any functional change; note that the actual
compiler-rt linking is handled by HandleCompilerRT.

Differential Revision: https://reviews.llvm.org/D58084

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@353786 91177308-0d34-0410-b5e6-96231b3b80d8