rpctls_impl.c: Use a direct cast to uintptr_t instead of __DECONST

[jhb]

Summary

This fixes the build on CHERI architectures where the compiler warns
about a direct cast between uint64_t and const void * inside of
DECONST. However, GCC would also complain about this on 32-bit
kernels due to uint64_t not being the same size as a pointer. Also,
a direct cast to uintptr_t to right-size the cookie value is more
direct than using DECONST given that there is no "const" pointer
involved in the expression.

Obtained from: CheriBSD
Sponsored by: AFRL, DARPA