Manifest: Mark Bls12 as unavailable on x86_32 and arm_32

Context

This merge request updates the manifest to allow specifying the availability of a package. The following availability can be added:

• Always: The package is always available. No entry will be added to the opam file.
• Never: The package will be marked as unavailable.
• No_32: The package is marked as unavailable on all 32-bits architectures.
• No_x86: The package is marked as unavailable on x86 architectures.
• No_ppc: The package is marked as unavailable on ppc architectures.
• No_arm: The package is marked as unavailable on arm architectures.
• List [a1;...;an]: The package availability is a1 & ... & an.

It also marke bls12_381 as unavailable on 32-bits architectures, ppc and s390x.

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR