Improve CMake handling of sse2 and sse3 and fix compile error

[larroy]

Description

This fixes a problem when compiling when F16C is enabled but the flag is missing for the compiler.
Fix #14304 where CFLAGS were overriden instead of appended to.
Improve CMake handling of sse2 and sse3

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

• The PR title starts with [MXNET-$JIRA_ID], where $JIRA_ID refers to the relevant JIRA issue created (except PRs with tiny changes)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage:
• Unit tests are added for small changes to verify correctness (e.g. adding a new operator)
• Nightly tests are added for complicated/long-running ones (e.g. changing distributed kvstore)
• Build tests will be added for build configuration changes (e.g. adding a new build option with NCCL)
• Code is well-documented:
• For user-facing API changes, API doc string has been updated.
• For new C++ functions in header files, their functionalities and arguments are documented.
• For new examples, README.md is added to explain the what the example does, the source of the dataset, expected performance on test set and reference to the original paper if applicable
• Check the API doc at http://mxnet-ci-doc.s3-accelerate.dualstack.amazonaws.com/PR-$PR_ID/$BUILD_ID/index.html
• To the my best knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change

[larroy]

@mxnet-label-bot add [CMake, build, Bug]

[larroy]

@mxnet-label-bot add [pr-awaiting-review]

[larroy]

@iblis17 any ideas?

[iblislin]

I rewrite that flaky test case: #14776

[larroy]

@lebeg please help review.

[stsukrov]

LGTM

[lebeg]

It would be better to have a separate module for SSE in general, but it looks good now as well