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

(cherry picked from commit bce0bef3c6abab92c7ac8cc23b7cc632a382721e)

libc: Add test cases for N2680.

This adds test cases for %wN and %wfN to the printf(3) and scanf(3) tests.

While here, fix a few nits in the N2630 test cases.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D41743

(cherry picked from commit 12b1c1e3fb446021a881d9815465137843fca50b)

Approved by: re (gjb)