gh-123748: Disable HACL SIMD256 on universal builds.

[freakboy3742]

The native HACL implementation added in #119316 compiles fine on single architecture x86_64 and ARM64 macOS builds, but failed when compiled on universal2 builds. This is because the autoconf detection is based on the existence of the -mavx2 compiler flag; this flag is legal in universal builds (because the x86_64 compilation path supports the flag), but this then requires symbols that aren't available on ARM64 builds. A standalone ARM64 build disables this feature.

This PR modifies the autoconf check so that if -mavx2 support is detected and the build is universal2, the autoconf feature isn't enabled.

• Issue: Error compiling HACL* Blake2 support for macOS universal binaries #123748

[ned-deily]

I agree with the subsequent discussion on the issue that using some sort of compile-time #define check would be better than trying to handle this solely in configure.

[freakboy3742]

Ok - closing on the basis that a #define approach is preferred; I'll try to get a separate PR up for that.