[secp256k1] Set ARM ASM symbol visibility to hidden

Summary:
This is a backport of secp256k1#1242
Co-authored-by: Cory Fields <cory-nospam-@coryfields.com>

Test Plan: push to github, run Github Actions (there are ARM builds)

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D19846