[libc] enable s(n)printf without fullbuild

To use the FILE data structure, LLVM-libc must be in fullbuild mode
since it expects its own implementation. This means that (f)printf can't
be used without fullbuild, but s(n)printf only uses strings. This patch
adjusts the CMake to allow for this.

Reviewed By: sivachandra, lntue

Differential Revision: https://reviews.llvm.org/D131913