make all_reference_decoders fails for Linux/arm64

[lumag]

Running make all_reference_decoders produces a lot of errors on Linux/arm64 machines:

• gcc-multilib / g++-multilib / passing -m32 to compiler is unavailable
• In a lot of cases decoder sources assume that CMAKE_SYSTEM_PROCESSOR is set to arm64 on non-x86 machines, however on Linux/arm64 this variable is set to uname -m, aarch64
• In some cases -msse4.1 is enabled unconditionally, even without the arch check

[mdimopoulos]

Hi and thanks for raising the issue. We have never used the provided Makefile in ARM64 to build reference decoders, it was created with x86_64 systems and Linux in mind (that part is covered by CI test workflow). It is a helper for developers and has never been part of the core features of fluster.
To put it in other words, the Makefile is provided "as is" without guarantee that it will work in the developer's system. All we can say is that it works on x86_64 with Ubuntu 20.04 and 22.04.

Let's leave the issue open, in case we decide to develop the Makefile further and add support for ARM64 with Linux and Apple Silicon with MacOS

[ylatuya]

This MR should address building the reference decoders for wider range of platforms and architectures: #324