Update BIKE documentation to exclude x86

[SWilson4]

We don't enable BIKE in x86 builds, and enabling it on x86 leads to CI failures. This PR is to bring documentation in line with the build configuration.

Fixes #1443.

Notably, the upstream implementation does apparently support x86, so this is something we may wish to look into.

• Does this PR change the input/output behaviour of a cryptographic algorithm (i.e., does it change known answer test values)? (If so, a version bump will be required from x.y.z to x.(y+1).0.)
• Does this PR change the list of algorithms available -- either adding, removing, or renaming? Does this PR otherwise change an API? (If so, PRs in fully supported downstream projects dependent on these, i.e., oqs-provider and OQS-OpenSSH will also need to be ready for review and merge by the time this is merged.)
  No to both.

[SWilson4]

I've opened #1678 to track the inconsistency with upstream.