ARC: rename arch_switch implementation to z_arc_switch

"arch_switch" is declared as an inline function in kswap.h,
it should be a wrapper of arch level switch. The difference
of declaration and implementation of "arch_swich" causes
warning from MWDT compiler.

Use "arch_switch" with proper declararion (which is just
wraper for "z_arc_switch") to do conext switch for ARC.

Signed-off-by: Wayne Ren <wei.ren@synopsys.com>
Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>