If Eviolite if attached to Zekrom, would its "Bolt Strike" attack do 40 or 20 damage to itself?
Eviolite reduces any damage done to the Basic Pokemon by attacks from either player. So Bolt Strike's self-damage would also be reduced by -20.
Source: Noble Victories FAQ (2011-11-17), PUI Rules Team (2011-11-17)

Relevant cards

Note: All applicable cards may not be listed below.
BW3_91
BW8_122
BW1_114
BW4_50