MFC: r344062 (partial)

• For diff reduction, bring in as much of the taskqgroup_attach{,_cpu}(9) fix from r344062 as possible without breaking KBI, which in turn means that these functions still don't properly work across architectures in stable/12 (in theory, compat shims should be possible but result in a PITA to maintain). However, e. g. the static iflib_irq_set_affinity() now also takes the irq as an if_irq_t instead of an int.
• Move the gtaskqueue_enqueue_fn typedef from <sys/gtaskqueue.h> to the gtaskqueue implementation as it's only used and needed there.
• Change the GTASK_INIT macro to use "gtask" rather than "task" as argument given that it actually operates on a struct gtask rather than a struct task.
• Let subr_gtaskqueue.c consistently use func to print functions names.