[CMake][libc] Fix non-runtime build when other runtimes are enabled

Before this patch, when other runtimes were enabled by setting
LLVM_ENABLE_RUNTIMES and llvm libc was built as a project by setting
LLVM_ENABLE_PROJECTS, the llvm libc CMake system would delay
configuration until the runtime build which never started since libc
wasn't declared as one of the runtime builds. This patch fixes this
behavior by explicitly checking that libc is within LLVM_ENABLE_RUNTIMES
rather than just the variable being set at all.

Reviewed By: Intue

Differential Revision: https://reviews.llvm.org/D151011