DSPHLE: Minor cleanup & savestate fix for AX coefficients.

[AdmiralCurtiss]

Fixes audio breaking if you load a state while a non-AX ucode is running.

[leoetlino]

Reviewed 2 of 2 files at r1, 1 of 1 files at r2, 2 of 2 files at r3, 4 of 4 files at r4, all commit messages.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @AdmiralCurtiss)

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Source/Core/Core/HW/DSPHLE/UCodes/AX.h, line 100 at r4 (raw file):

  // The coefficients aren't always available (they are part of the DSP DROM)
  // so we also need to know if they are valid or not.
  bool m_coeffs_available = false;

Thoughts on combining m_coeffs_available and m_coeffs_checksum into a std::optional m_coeffs_checksum? That would simplify the check in the savestate code and would make it impossible to forget checking or updating available/checksum.

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Source/Core/Core/HW/DSPHLE/UCodes/AX.cpp, line 72 at r4 (raw file):

      continue;

    std::memcpy(m_coeffs.data(), raw_coeffs.data(), raw_coeffs_size);

#include <cstring>

[leoetlino]

Reviewed 3 of 3 files at r5, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @AdmiralCurtiss)