[libc] Add a JSON based config option system.

Few printf config options have been setup using this new config system
along with their baremetal overrides. A follow up patch will add generation
of doc/config.rst, which will contain the full list of libc config options
and short description explaining how they affect the libc.

Reviewed By: gchatelet

Differential Revision: https://reviews.llvm.org/D159158