os-wrappers-test.c: Correctly forward arguments to fcntl

We can't just unconditionally read the optional arguments (and also read
it as a void* despite actually being an int).
While this happens to work on most architectures because the first few
variadic arguments are passed in registers, this is non-portable and
causes a crash on architectures that set bounds on variadic function
arguments (for example CHERI-enabled architectures). It could also cause
problems on big-endian architectures that pass variadic arguments on the
stack rather than in registers.

For CHERI-MIPS, reading sizeof(void*) causes a read of 16 bytes from the
bounded varargs capability. This always crashes since even calls with the
optional argument only have 4 bytes available.

Signed-off-by: Alex Richardson <Alexander.Richardson@cl.cam.ac.uk>
Reviewed-by: Simon Ser <contact@emersion.fr>