MFH: r553707

emulators/mgba: Update to 0.8.4

ChangeLog: https://mgba.io/2020/10/29/mgba-0.8.4/

PR: 250734
Submitted by: james.wright@digital-chaos.com
Approved by: greg@unrelenting.technology (maintainer)

Approved by: ports-secteam (blanket, runtime fix)