kernel: mark z_smp_current_get() with the const attribute

Repeated references to _current won't produce a different result as the
executing thread instance is always the same. Use the const attribute to
let the compiler know it may reuse a previously obtained value. This offset
the penalty for moving z_smp_current_get() out of line and provides yet
more binary size reduction.

This change is isolated in its own commit to ease bisecting in case some
unexpected misbehavior is eventually observed.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>