[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