cleanup platform support functions

[franziskuskiefer]

          `libcrux_platform` checks for `avx` in simd128_support() besides `avx2` in simd256_support(), AFIAK both aren't supported in x86 32-bit processors but can be enabled with `-m32` mode on x64 targets

Originally posted by @mamonet in #73 (comment)

Also: #73 (comment)

[github-actions]

This issue has been marked as stale due to a lack of activity for 60 days. If you believe this issue is still relevant, please provide an update or comment to keep it open. Otherwise, it will be closed in 7 days.

[github-actions]

This issue has been closed due to a lack of activity since being marked as stale. If you believe this issue is still relevant, please reopen it with an update or comment.