MySQL dialect must still work in sql_mode=oracle

All MySQL-specific dialect expressions must work in sql_mode=oracle , (unless they really conflict with Oracle syntax - but then it may be serious limitation for some cases).

The reasoning behind that is that some 10.2 environments may rely on that dialect with sql_mode=oracle and it may be very unfortunate if those stop working after upgrading to 10.3 .
As example I could identify following cases in MDEV-12962 :

1. ELSEIF stopped working in sql_mode=oracle, which recognising only ELSIF now
2. '%' operator has gone in sql_mode=oracle (see MDEV-12977)

Other cases must be considered as well in 10.3 oracle parser during testing / implementation.