[CMake] Don't use -rtlib=compiler-rt with -nodefaultlibs.

Summary:
This switch only has an effect at link time. It changes the default
compiler support library to `compiler-rt`. With `-nodefaultlibs`, this
library won't get linked anyway; Clang actually warns about that.

Reviewers: mstorsjo, rnk

Subscribers: dberris, mgorny, christof, cfe-commits

Differential Revision: https://reviews.llvm.org/D51645

llvm-svn: 341404