Misc: Code janitorial work

[stenzek]

Description of Changes

• Moves a few things over to C++20 replacements, rather than the wrappers we currently use for compatibility.
• Simplify the assertions, pxAssertDev and pxAssertMsg were basically the same, so collapse that back into one.
• Replace the generic CRT assertions with pxAssert (only affects devbuilds, previously they wouldn't trigger).
• Cut out a lot of the cruft in Pcsx2Defs.h, mainly irrelevant platforms/compilers.
• Remove explicit PrecompiledHeader.h includes, the build system force includes it.
• Simplify the intrinsic file includes for future platforms.
• Add all the missing includes that were indirectly pulled in from PrecompiledHeader.h for pch-disabled builds.

Since I was editing most of the cpp files anyway, I replaced all the old license headers with small and compact SPDX headers.

Rationale behind Changes

No functional changes, but general cleanup.

Suggested Testing Steps

Test compressed games (CHD/CSO).
Make sure performance hasn't changed in HW and SW.