Bug#761300: libc6: putchar does not follow stdio

[Florian Weimer <fw@deneb.enyo.de>]

* Sven Joachim:

> Am 14.10.2018 um 13:38 schrieb Florian Weimer:
>
>> * Sven Joachim:
>>
>>> This result is rather surprising.  After all, "putchar('x')" is supposed
>>> to do the same as "putc('x', stdout)", but here it does not.
>>
>> Can you reproduce this with something newer than 2.13-38+rpi2+deb7u3?
>> Or on something else besides armhf?
>
> Surely, I tested 2.27-6 on amd64.

Eh, right.  We have this in glibc:

int
putchar (int c)
{
  int result;
  _IO_acquire_lock (_IO_stdout);
  result = _IO_putc_unlocked (c, _IO_stdout);
  _IO_release_lock (_IO_stdout);
  return result;
}

_IO_stdout is the variable to which stdout points to default, so this
code was simply not adjusted when the assignment-to-stdout extension
was implemented.  This should use stdout instead.  The puts function
has the same problem.

It looks to me we can get away with fixing this bug.