remove flags

[meiravgri]

This PR removes the following global compiler flags from the CMake configuration:

-march=native -mtune=native

These flags were originally introduced in [#598](#598) to enable high-performance execution of the SVS dependency by leveraging the build machine's full CPU capabilities.

However, applying -march=native and -mtune=native globally causes several issues:

• Cross-device compatibility: hese flags enforce optimizations based on the build machine's architecture, making the resulting binaries potentially incompatible with machines of different architectures.

• Conflicts with file-specific flags: Some files require specific architecture features. For example, ARM files using intrinsics like vdotq_s32 are explicitly compiled with -march=armv8.2-a+dotprod. Despite this, the presence of -mtune=native breaks the build. Removing it resolves the issue. This may be a bug in how GCC chains -mtune=native in combination with specific -march= overrides, as discussed here https://bugs.gentoo.org/884549.

• Inconsistent optimization strategy: In VecSim, architecture-specific optimizations are selected dynamically at runtime using a dispatcher based on the actual CPU running the binary. This pattern is also being extended for SVS in [intel/SVS PR #113](CPUArch dispatching and unified shared library intel/ScalableVectorSearch#113).

Summary

We are removing -march=native and -mtune=native from the global CMake configuration to:

• They lock in optimizations for the build machine’s CPU, breaking portability.
• They interfere with files requiring explicit -march values.
• They are incompatible with our runtime architecture selection strategy.

This change ensures that:

• The global build remains architecture-agnostic and portable.
• File-specific or runtime-selected optimizations can be applied safely where needed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.24%. Comparing base (98cf0a1) to head (68a37a1).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #671   +/-   ##
=======================================
  Coverage   96.24%   96.24%           
=======================================
  Files         112      112           
  Lines        6278     6278           
=======================================
  Hits         6042     6042           
  Misses        236      236           

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