fiber: introduce a callback for fibers switch-over

Tarantool integrates several complex environments together and there are
issues occurring at their junction leading to the platform failures.
E.g. fiber switch-over is implemented outside the Lua world, so when one
lua_State substitutes another one, main LuaJIT engines, such as JIT and
GC, are left unnotified leading to the further platform misbehaviour.

To solve this severe integration drawback <cord_on_yield> function is
introduced. This routine encloses the checks and actions to be done when
the running fiber yields the execution.

Unfortunately the way callback is implemented introduces a circular
dependency. Considering linker symbol resolving methods for static build
an auxiliary translation unit is added to the particular tests mocking
(i.e. exporting) <cord_on_yield> undefined symbol.

Part of #1700
Relates to #4491

Reviewed-by: Sergey Ostanevich <sergos@tarantool.org>
Reviewed-by: Vladislav Shpilevoy <v.shpilevoy@tarantool.org>
Signed-off-by: Igor Munkin <imun@tarantool.org>