libc: Implement N2680.

This adds specific width length modifiers in the form of wN and wfN (where N is 8, 16, 32, or 64) which allow printing intN_t and int_fastN_t without resorting to casts or PRI macros.

Reviewed by: imp, emaste
Differential Revision: https://reviews.freebsd.org/D41725