build: compile Firecracker with retpoline

[bchalios]

Changes

Add rustc features retpoline-indirect-branches and retpoline-indirect-calls to compile Firecracker itself with retpoline mitigations for Spectre attacks

Reason

Wanna try out the performance penalty we get when we enable retpoline

License Acceptance

By submitting this pull request, I confirm that my contribution is made under
the terms of the Apache 2.0 license. For more information on following Developer
Certificate of Origin and signing off your commits, please check
CONTRIBUTING.md.

PR Checklist

• I have read and understand CONTRIBUTING.md.
• I have run tools/devtool checkbuild --all to verify that the PR passes
  build checks on all supported architectures.
• I have run tools/devtool checkstyle to verify that the PR passes the
  automated style checks.
• I have described what is done in these changes, why they are needed, and
  how they are solving the problem in a clear and encompassing way.
• I have updated any relevant documentation (both in code and in the docs)
  in the PR.
• I have mentioned all user-facing changes in CHANGELOG.md.
• If a specific issue led to this PR, this PR closes the issue.
• When making API changes, I have followed the
  Runbook for Firecracker API changes.
• I have tested all new and changed functionalities in unit tests and/or
  integration tests.
• I have linked an issue to every new TODO.

---

• This functionality cannot be added in rust-vmm.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.35%. Comparing base (d974044) to head (0db9630).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5384      +/-   ##
==========================================
+ Coverage   82.31%   82.35%   +0.04%     
==========================================
  Files         265      265              
  Lines       30645    30645              
==========================================
+ Hits        25224    25238      +14     
+ Misses       5421     5407      -14     

Flag	Coverage Δ
5.10-c5n.metal	82.32% <ø> (ø)
5.10-m5n.metal	82.32% <ø> (+<0.01%)	⬆️
5.10-m6a.metal	81.60% <ø> (-0.01%)	⬇️
5.10-m6g.metal	78.92% <ø> (ø)
5.10-m6i.metal	82.32% <ø> (ø)
5.10-m7a.metal-48xl	81.59% <ø> (?)
5.10-m7g.metal	78.92% <ø> (ø)
5.10-m7i.metal-24xl	82.29% <ø> (?)
5.10-m7i.metal-48xl	82.29% <ø> (?)
5.10-m8g.metal-24xl	78.92% <ø> (?)
5.10-m8g.metal-48xl	78.92% <ø> (?)
6.1-c5n.metal	82.37% <ø> (+<0.01%)	⬆️
6.1-m5n.metal	82.36% <ø> (ø)
6.1-m6a.metal	81.65% <ø> (ø)
6.1-m6g.metal	78.92% <ø> (ø)
6.1-m6i.metal	82.36% <ø> (ø)
6.1-m7a.metal-48xl	81.64% <ø> (?)
6.1-m7g.metal	78.92% <ø> (ø)
6.1-m7i.metal-24xl	82.38% <ø> (?)
6.1-m7i.metal-48xl	82.38% <ø> (?)
6.1-m8g.metal-24xl	78.92% <ø> (?)
6.1-m8g.metal-48xl	78.92% <ø> (?)

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