DSPHLE MailHandler: Synchronize reads and interrupts
This change is meant to solve the following problem: how to translate the
following snippet to DSPHLE:
SendInterruptAndWaitRead(MAIL_A);
SendAndWaitRead(MAIL_B);
SendInterruptAndWaitRead(MAIL_C);
This should cause the following actions on the CPU side:
---> Woken up by interrupt
Reads MAIL_A
Reads MAIL_B
<--- Exits interrupt handler
---> Woken up by interrupt
Reads MAIL_C
<---
But with the current D