rb tree: remove strict aliasing violations

Rework internal RB macros to avoid assignments via type punned pointers.
RB uses low order pointer bits to encode information (whether children
are red), and was manipulating those values via (*(__uintptr_t *)&elm),
which leads to strict aliasing warnings.

In the kernel we use -fno-strict-aliasing, but this isn't necessarily
the case in user space. This quiets thousands of -Wstrict-aliasing
warnings in the user space build.

Reported by: GCC -Wstrict-aliasing
Reviewed by: dougm
Discussed with: kib
Differential Revision: https://reviews.freebsd.org/D52939