xtensa: ISS: fix __simc implementation

Do not rely on compiler implicitly placing function arguments into
specifc registers, place them explicitly.

Move __simc and related helpers to platform/simcall.h

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Chris Zankel <chris@zankel.net>
3 files changed