znver3 tuning part 1

[Jan Hubicka]

> 
> 2021-03-15  Jan Hubicka  <hubicka@ucw.cz>
> 
> 	* config/i386/i386-options.c (processor_cost_table): Add znver3_cost.
> 	* config/i386/x86-tune-costs.h (znver3_cost): New gobal variable; copy
> 	of znver2_cost.

I have backported the patch to gcc10 branch as
g:aa99212489545c6c970a8f91b3d37ea6466cb988.

Honza