Mark getc_unlocked as unavailable by default

Before D45736, getc_unlocked was available by default, but turned off
for non-Cygwin/non-MinGW Windows. D45736 then added 9 more unlocked
functions, which were unavailable by default, but it also:
 * left getc_unlocked enabled by default,
 * removed the disabling line for Windows, and
 * added code to enable getc_unlocked for GNU, Android, and OSX.

For consistency, make getc_unlocked unavailable by default. Maybe this
was the intent of D45736 anyway.

Reviewed By: MaskRay, efriedma

Differential Revision: https://reviews.llvm.org/D107527