Fix a possible deadlock.

If fwrite is called, a lock might be acquired for the file resource as the buffer is updated.
This leads to a deadlock, e.g. if fwrite prints to stdout/err and a warning is triggered as the lock is acquired again.

Replace fprintf with klee_warnings for POSIX lib to avoid the runtime library and use KLEE functionality instead.
Fixes test/Runtime/POSIX/Write2.c bc177e9d48719da65d65d76e4c9203f45876b125