libc: provide fputc_unlocked

Among the same justification as the other stdio _unlocked; in addition to an
inline version in <stdio.h>, we must provide a function in libc as well for
the functionality. This fixes the lang/gcc* builds, which want to use the
symbol from libc.

PR: 243810
Reported by: antoine, swills, Michael <michael.adm gmail com>
X-MFC-With: r357284