kernel: spinlock: k_spin_is_locked introduction

Currently spinlock internals are directly accessed from the tests.
This way the test becomes bound to the particular spinlock implementation.
To remove this unnecessary dependency the distinct API to check if spinlock
is locked is introduced.

k_spin_is_locked should be used for the spinlock testing only,
so the scope of this API is intentionally restricted.

Signed-off-by: Alexander Razinkov <alexander.razinkov@syntacore.com>