Make __sys_fcntl take an intptr_t

[brooks]

Summary

The (optional) third argument of fcntl is sometimes a pointer so change
the type to intptr_t. Update the libc-internal defintion (actually used
by libthr) to take a fixed intptr_t argument rather than pretending it's
a variadic function. (That worked because all supported architectures
pass variadic arguments as though the function was declared with those
types. In CheriBSD that changes because variadic arguments are passed
via a bounded array.)