These values were obtained by setting a breakpoint at a game's entry point, and then observing the register values with Dolphin's register widget.
There are other registers that aren't handled by this PR, including CR, XER, SRR0, SRR1, and "Int Mask" (as well as most of the GPRs). They could be added in a later PR if it turns out that their values matter, but probably most of them don't.
This fixes Datel titles booting with the IPL skipped (see https://bugs.dolphin-emu.org/issues/8223), though when booted this way they are currently missing textures. Due to somewhat janky code, Datel overwrites the syscall interrupt handler and then immediately triggers it (with the `sc` instruction) before they restore the correct one. This works on real hardware due to icache, and also works in Dolphin when the IPL runs due to icache, but prior to this change `HID0.ICE` defaulted to 0 so icache was not enabled when the IPL was skipped.
DSPHLE::Initialize sets the halt and init bits to true (i.e. m_dsp_control.Hex starts as 0x804), which is reasonable behavior (this is the state the DSP will be in when starting a game from the IPL, as after `__OSStopAudioSystem` the control register is 0x804).
However, CMailHandler::m_halted defaults to false, and we only call CMailHandler::SetHalted in DSPHLE::DSP_WriteControlRegister when m_dsp_control.DSPHalt changes, so since DSPHalt defaults to true, if the first thing that happens is writing true to DSPHalt, we won't properly halt the mail handler.
Now, we call CMailHandler::SetHalted on startup. This fixes Datel titles when the IPL is skipped with DSP HLE (though this configuration only works once https://bugs.dolphin-emu.org/issues/8223 is fixed).
This fixes booting Datel titles with DSPHLE (see https://bugs.dolphin-emu.org/issues/12943). Datel messed up their DSP initialization code, so it only works by receiving a mail later on, but if halting isn't implemented then it receives the mail too early and hangs.
It's cleared whenever the uCode changes, so there's no reason to clear it in a destructor or during initialization.
I've also renamed it to ClearPending.
The # option means that 0x is prepended already, so the old code resulted in 0x0xDEADBEEF instead of the intended 0xDEADBEEF. WriteMailboxLow was already correct.
