Fix compiling for Arm64 Windows with MSVC

[aaronfranke]

This PR fixes 3 issues when compiling for Arm64 Windows with MSVC.

• Fix detection of being a 64-bit platform. I replaced the defined(__X86_64__) || defined(__aarch64__) check with a platform-agnostic UINTPTR_MAX == UINT64_MAX that relies on stdint.h (please let me know if any of Embree's targets do not have stdint.h, my assumption is that this is fine to depend on, but I know that MSVC versions from before 2010 don't have it). MSVC does not define __aarch64__, but it does define _M_ARM64. Adding || defined(_M_ARM64) would also fix the problem, but it's best to make this platform-agnostic.
• Always define unsigned 64-bit versions of bsf, bscf, and bsr. Without it, this function was not being defined on MSVC. Same as the above point, MSVC does not define __aarch64__, but it does define _M_ARM64. Adding || defined(_M_ARM64) would also fix the problem, but it's best to make this platform-agnostic. Using size_t with #if guards for 64-bit is the same as just using uint64_t. Note that functions may be automatically removed by the compiler if unused, so there is no reason to explicitly wrap it in #if guards.
• Before #include <immintrin.h>, check if _M_ARM64 is defined (meaning Arm64 Windows MSVC) and if so define USE_SOFT_INTRINSICS. MSVC provides immintrin.h with software support on Arm64, but this has to be enabled explicitly. The MSVC-provided immintrin.h contains this code:

#if !defined(_M_IX86) && !defined(_M_X64) && !(defined(_M_ARM64) && defined(USE_SOFT_INTRINSICS))
#error This header is specific to X86, X64, ARM64, and ARM64EC targets
#endif

[aaronfranke]

@freibold This PR is needed to fix Arm64 Windows builds with MSVC. Let me know if I can do anything to push this forward.

[freibold]

Hi @aaronfranke! Thanks for this PR. I had time to look at this end of last week. However, I had issues getting this to work using Visual Studio 2022 "ARM edition". It was compiling but the linker complained that it can't find "__mm_load_ps", "mm_store_ps", and other SSE intrinsic. I was hoping that adding the missing USE_SOFT_INTRINSICS in common/sys/thread.cpp would fix the issue but no luck so far. Did you add any additional CMake or CXX flags on your side?

On a more high-level I don't think the software intrinsic are the way to go. I was trying to enable ARM NEON via the SSE2NEON "library" like we do for Apple ARM but I hit a roadblock there, too. However, I think ARM NEON is the right for performance reasons. Anyhow, if you can help me out with getting your USE_SOFT_INTRINSICS solution to work, I'm happy to merge that in as a short-term Windows ARM solution.

[aaronfranke]

@freibold Actually, that was where I got stuck too, it fails at the linker. godotengine/godot#91360 (comment)

At the time I submitted this PR (and at the time I wrote the ping to you) I had only tested compiling before the linker.

You are likely correct that the USE_SOFT_INTRINSICS stuff is completely wrong, but I believe the bsf, bscf, and bsr changes are correct, and the #if UINTPTR_MAX == UINT64_MAX change too. Hopefully this PR would be a good base for building a solution using Arm Neon. I'm completely lost when it comes to this, however. If you want I can remove the USE_SOFT_INTRINSICS from this PR so you can merge in the rest of the changes.