Specify C compiler in build cross-compilation

[tarakby]

Closes #534

Description

• remove unused Dockerfile.relic
• update Dockerfile go build step:
  • enable cgo.
  • specify the C compiler for each arch target. This insures cross-compilation of the crypto non-portable code succeeds.
  • disable ADX instructions when compiling the crypto lib: crypto operations become slower but it makes sure binary can run on all CPUs.

Builds were enabled temporarily while working on the PR. You can see the results here.

---

For contributor use:

• Targeted PR against master branch
• Linked to GitHub issue with discussion and accepted design OR link to spec that describes this work
• Code follows the standards mentioned here
• Updated relevant documentation
• Re-reviewed Files changed in the GitHub PR explorer
• Added appropriate labels

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (91908ef) 53.68% compared to head (4ff4de9) 53.68%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #548   +/-   ##
=======================================
  Coverage   53.68%   53.68%           
=======================================
  Files          31       31           
  Lines        3962     3962           
=======================================
  Hits         2127     2127           
  Misses       1672     1672           
  Partials      163      163           

Flag	Coverage Δ
unittests	53.68% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[tarakby]

Builds were enabled temporarily while working on the PR. You can see the results here.

[turbolent]

LGTM