8-bit Normal Serial IO mode not working

[felixjones]

ELF & ROM download (project.zip)
((please excuse the use of "master/slave" terminology))

On mGBA the master player must await for the ready state of the slave player. This is done by reading bit 2 of REG_SIOCNT (AKA "SI State (opponents SO)") until the value is "low" (0, ready state).

This status never changes in mGBA. It is always "high" (1, not ready state).

Video of the above ROM on hardware:
https://user-images.githubusercontent.com/1824671/118163070-d4ad9c00-b421-11eb-96ee-9a2be376da84.mp4

In this demo ROM player 1 can send buttons A and/or B to player 2 (regardless of master/slave setting, however P2 cannot send to P1 - due to normal mode being one directional with AGB cables. CGB cables are two directional).

Tested on latest mGBA as of this writing (0.10-6995-96988133b)
Recreation steps:

• Load up ROM
• Open multiplayer window
• Load up same ROM
• Select Master on Player 1
• Select Slave on Player 2
• Press A and/or B on Player 1
• Expected: Player 2 receives value. Actual: Master "times out" (bit 2 of REG_SIOCNT never changes).

EDIT: Tested with Normal32, which seems to work fine. I suppose this issue is Normal8 only.