[libc] Include (v)s(n)printf in baremetal configs

These are commonly used on baremetal targets. We disable float
support and other features to reduce the binary size. This would
ideally eventually be handled using the proposed config mechanism:
https://discourse.llvm.org/t/rfc-systematic-way-to-introduce-and-use-libc-config-options/72943
but for now we use a CMake conditional.

Differential Revision: https://reviews.llvm.org/D159067