BUG/MEDIUM: tasks: don't type-pun state and tid in builds without threads

BUG/MEDIUM: tasks: don't type-pun state and tid in builds without threads

__task_set_state_and_tid() updates t->state and t->tid at once through
an uint64_t cast, which violates strict aliasing. With threads the
__atomic builtins keep the compiler conservative, but without them the
accesses are plain, and gcc -O2 legitimately keeps using a stale
t->state: the CAS always fails and _task_wakeup() spins forever on the
first request until the watchdog kills the process.

Without threads there is no concurrency to protect against, so just
compare and assign the two fields directly.