Allow field_10x26_arm.s to compile for ARMv7 architecture

Summary: This is a backport of secp256k1's PR612

Test Plan: I do not have an arm device where I can test this on.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4992