[linux kernel code] CPU feature detection fixups (BMI2)

FYI as this may need to be synced back to base zstd code. There was some discussion how CPU features should be detected in ZSTD code, moving to detecting support only once and using only the kernel helpers

https://lore.kernel.org/all/CAHk-=whZf9=X+qKkO9_pQzbLdOnbMw5yO8gu0TkahegVW3cC-g@mail.gmail.com/

The patchset with latest version (v2):

https://lore.kernel.org/all/20260830222100.2706175-1-usama.arif@linux.dev/