[SECP256K1] Turn off ASM by default on target with no ASM support

Summary:
This will drop the need to set -DSECP256K1_USE_ASM=OFF for targets
with no support for ASM in secp256k1.

Test Plan:
Run cmake on various platforms and check the ASM is enabled/disabled as
needed.

Run the Gitian builds.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5590