Deprecate the ARM (32 bits) variant

Summary:
This is no longer used these days, and only some Raspberry Pi still use this. Since 64 bit OSes are also available for these boards, it's time to deprecate the last remaining 32 bits architecture that is still supported.

Note: the dependencies and build files are still available for users willing to build themselves, but this will no longer be tested/maintained. This code might be removed in the future if there is no interest in keeping them.

Depends on D16903.

Test Plan: Grep for missed references

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16902