Add the THREADS_PREFER_PTHREAD_FLAG explicitly for zig builds;
these are the only two ports that need that, otherwise it would
be a candidate for adding to our platform CMake flags.
Without the flag, CMake 3.24 changes how pthreads are searched,
and it ends up leaving out the -pthread flag from the compile.
For zig, this causes the bootstrap compiler to crash immediately
(it does not even reach main), which kills the build. Prior to
CMake 3.24, this flag already existed, but the order in which
pthreads were searched was different, and we ended up using the
-pthread flag anyway.
The PR is for the CMake update. This was the last blocker for it.
PR: 265652
bdc2b54bd1