tests/kernel/smp: Misc synchronization fixups

A few mistakes in recent changes to this test:

There was a "LOCK_NO" (i.e. no locking!) case being exercised in
test_inc_concurrency, where three threads would race against each
other incrementing and decrementing a single count without
synchronization.  And... it failed on cAVS.  Because there was no
synchronization.  Just remove.

The LOCK_IRQ (irq_un/lock()) case of the same test was was casting
taking a pointer to an integer (that stored the irq_lock() result) and
casting the pointer value to an integer instead of dereferencing it.

Also the workq test had a work item on the stack, which is forbidden
when KERNEL_COHERENCE=y

Fixes #34152

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>