GS Include Cleanup

[TellowKrinkle]

Description of Changes

• Move preprocessor macros to a header that's included by everything, to prevent accidents with ifdef
• Remove custom countof macros in favor of std::size (or for-in loops / std::fill where applicable)
• Replace GS's integer typedefs with the ones used by the rest of PCSX2 (uint32, int64 → u32, s64)
  • Note: A number of structs had conflicting members (e.g. GSVector4::u32), I fixed that by renaming those to use capital letters
• Split GS.h into multiple smaller headers
• Stop pretending GS.h is a PCH

Note: Builds upon #4708

Rationale behind Changes

• Cleaner, more consistent code
• Better compile times and less files recompiled per change in a header

Suggested Testing Steps

Make sure everything builds