MFC r357284, r357419: stdio unlocked

r357284:
stdio: provide _unlocked variants of fflush, fputc, fputs, fread, fwrite

fflush_unlocked is currently desired in ports by sysutils/metalog, and
redefined as the locked fflush.

fputc_unlocked, fputs_unlocked, fread_unlocked, and fwrite_unlocked are
currently desired in ports by devel/elfutils, and redefined as the locked
fputs, fread, and fwrite respectively.

r357419:
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.