fix Issue 10462 - call through function pointer might trash EBX

add EBX to the keepmsk after GOT was loaded

This is a followup to #2278. The fix feels a little hacky to me so please have a look @WalterBright whether it's appropriate.

[Bugzilla 10462](http://d.puremagic.com/issues/show_bug.cgi?id=10462)