Wrap attributes in platform checks to preserve portability

[StefanHamminga]

The inserted attributes prevent compilation on, for example, arm.

Wrapping the inserted line in a check for x86 platforms will preserve portability:

    ...
#if defined(i386__) || defined(__i386__) || defined(_M_IX86) || defined(_M_IX86) || defined(__X86__) || defined(_X86_) || defined(__I86__) || defined(__INTEL__) || defined(__amd64__) || defined(__amd64) || defined(__x86_64__) || defined(__x86_64) || defined(_M_AMD64)
    __attribute__((target_clones("avx2","arch=atom","default")))
#endif
    ...

Defines taken from: https://sourceforge.net/p/predef/wiki/Architectures/

[VictorRodriguez]

@StefanHamminga let me know if #11 is a way to address the portability

[StefanHamminga]

Excellent, works for me, thanks (both for addressing this and the work on this project)!