eo: Change eo_override to avoid struct passing and GCC construct

eo: Change eo_override to avoid struct passing and GCC construct

This is a (minor) API & ABI break in Eo.h!

I say minor as eo_override shouldn't be used yet (EO is unstable
and this patch includes all the use cases in EFL).

I'm not very happy about the new form of the macro, but it avoids
two things:
- passing in a struct (valid in C, but never used in EFL)
- using a GCC construct to create structs on the fly

It was inspired by the event array define, but I don't think
we need the runtime memcpy here.

See also:
https://gcc.gnu.org/onlinedocs/gcc/Compound-Literals.html