kernel/spinlock: Fix a SMP race condition of SPIN_VALIDATE

z_spin_lock_valid() reads shared variable twice to do two checkings. If
this variable is modified by other CPU between two read accesses, the
checking value is inconsistent. This inconsistency causes the error
that CPU0 can pass the checking when it doesn't hold spinlock because
zeroed-out thread_cpu value is ambiguous with the CPU0 ID.

Fix the inconsistency by only reading shared variable once and using
local variable value to do two checkings.

Fixes #19299.

Signed-off-by: Jim Shu <cwshu@andestech.com>