[backport] Scripts and tools: Fix BIND_NOW check in security-check.py

Summary:
37d363dd4acf7d41196d44b295593019add241c3 Tools: fix BIND_NOW check in security-check.py (Conrado Gouvea)

Pull request description:

Previously, the BIND_NOW check would work only if it was the first value in FLAGS.

Tree-SHA512: 39cd83f673a6b521803be5585ada516f2db4eede718f0c5aea3377825ed1adcefed5bbb41dd9a5f24a26f7d27116cfc81bde7e26283232593b72768c5ae3d321

This is a backport of Core PR13647

Test Plan:
compile a "Hello, world!" C program with both:

gcc helloworld.c -o test_normal
gcc -Wl,-z,now -fstack-protector-all helloworld.c -o test_bindnow_canary

running ./security-check.py test_normal returns test_normal: failed RELRO Canary
running ./security-check.py test_bindnow_canary returns nothing

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5709