[SECP256K1] Disable ASM for native executables

Summary:
If the platforms is detected x86_64 and the ASM check fails, the native
executable will fail as well even if -DSECP256K1_USE_ASM=OFF is passed
to cmake. This diff disables the ASM for the native executables and get
rids of the issue.

Got the issue on FreeBSD 12.0.

Test Plan:

ninja check

Build on FreeBSD 12.0.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5542