brz.c: Use uintptr_t instead of a hardcoded list of 64-bit arches

This also fixes the build for AArch64 since cmph_uint32 is not big enough to hold a pointer there. I found this while building for CHERI since the compiler errors there when casting from a non-inptr_t integer. This also fixes warnings when building for AArch64 with clang and most likely fixes previously broken code (it's unlikely all pointers are allocated in the low 32 bits).