POSIX: fixed hrt call and workqueue implementation

The HRT call processing normally happens via HW timer interrupt
handler. Since the POSIX port has no ISR handling, the HP work
queue is used.

Instead of irq_save() and irq_restore() calls to disable/enable
interrupts, a mutex is used to protect each queue.

Signed-off-by: Mark Charlebois <charlebm@gmail.com>