Allow building on non-x86 systems

[scaronni]

Checking if there is any interest for this... This patch allows to build for ARM systems without the need for a cross compilation on an x86_64 system.

The changes allow the software to build like a native package on an aarch64 system, pretty much like the x86_64 one.

Linux distributions shipping xevd patch the software in a similar way to have a "normal" package that builds from source.

If there is any interest, I can update the various docs on the topic.

[scaronni]

Works for both main and base profiles. I also have the equivalent xevd PR: mpeg5/xevd#87

[kpchoi]

Thanks for the patch, this is useful for distro packaging. I picked it up on the non_x86_build branch (https://github.com/mpeg5/xeve/tree/non_x86_build) with your commit kept as is, plus a follow-up commit fixing a few things found while testing:

• The generic (plain C) fallback did not build for the main profile: src_main/xevem_type.h still had #ifndef ARM around the SSE/AVX includes, so they were pulled in on non-x86/non-ARM targets. Changed to #if defined(X86).
• On Windows CMAKE_SYSTEM_PROCESSOR is AMD64 (or ARM64), which the case-sensitive regex did not match, so MSVC x64 builds would silently lose all SIMD. Detection is now case-insensitive and includes amd64.
• Moved the detection to the top-level CMakeLists so it runs once, applies -DX86/-DARM on all platforms including MSVC, and keeps the documented -DARM=TRUE cross-build flow working.
• Dropped the src_main/neon globs since that directory does not exist.

Verified on x86-64 (SIMD kept, bit-exact output) and with a forced riscv64 target for the C fallback, both profiles. Could you give the branch a try on your aarch64 setup? If it works for you we can merge it that way. Doc updates for the new behavior would be welcome too.

[kpchoi]

Opened #164 with your commit included as is (authorship preserved) plus the follow-up fixes mentioned above. Thanks a lot for the contribution — this makes packaging much easier for non-x86 distros. Testing on your aarch64 setup is still very welcome over there before we merge.