kernel/spinlock: Move validation out of header inlines

The validation checking recently added to spinlocks is useful, but
requires kernel-internals like _current and _current_cpu in a header
context that tends to be needed before those are declared (or where we
don't want them declared), and is causing big header dependency
headaches.

Move it to C code, it's just a validation tool, not a performance
thing.

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