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 xeve PR: mpeg5/xeve#158

[scaronni]

Anyone looking at pull requests?

[kpchoi]

Thanks, same as with the xeve counterpart (mpeg5/xeve#158) I picked this up on the non_x86_build branch (https://github.com/mpeg5/xevd/tree/non_x86_build) with your commit kept as is, plus a follow-up commit:

• The arch detection regex did not match AMD64/ARM64 (Windows), so MSVC 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 -DARM=TRUE cross-build flow working. This also removes the duplicate ARM block the patch added at the bottom of the top-level CMakeLists (one already existed above it).

Verified on x86-64 (SIMD kept) and with a forced riscv64 target for the plain C fallback, both profiles, decoded output bit-exact in all cases. Please give the branch a try on your aarch64 setup.

[kpchoi]

Opened #91 with your commit included as is (authorship preserved) plus the follow-up fixes mentioned above. Thanks a lot for the contribution, same as on the xeve side. Testing on your aarch64 setup is still very welcome over there before we merge.