Information for new-zelda-hle-4.0-4169

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