Fix flockfile(3) and friends for stdin/stdout/stderr too.

stdin/stdout/stderr are special; their mutexes are initialized by
__sinit. There's no unit test for this, because __sinit has already
been called by the time the first unit test runs, but you could
reproduce this failure with a trivial main() that calls flockfile
or ftrylockfile on one of the standard streams before otherwise
using stdio.

Bug: 18208568
Change-Id: I28d232cf05a9f198a2bed61854d8047b23d2091d