Make external interrupts thread-safe (fixes loss of audio on Super Mario Sunshine)

Issue 4940;  fixes the introduction of what seems to be a non-thread-safe call to ExpansionInterface::UpdateInterrupts() from PowerPC.cpp that was introduced back in the gc-mic branch merge, by making ExpansionInterface::UpdateInterrupts() thread-safe. Tested on two mid-range systems that reliably would completely lose audio on Super Mario Sunshine previously, and it's now rock-solid. Also confirmed the mic test in Mario Party 7 still works with this change.

Tested a few Wii and GC games with this change and no side effects so far that I can see.