libc/stdio: Increase BUF in vfprintf.c and vfwprintf.c

With the %b format specifier we need enough space to write a uintmax_t
in binary.

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1400

(cherry picked from commit d4f9e326393e3298062a58338e2c94ef6baff8b5)