improve dbg intrinsic with fflush

[xunilrj]

Description

This PR brings a new syscall "fflush" to help EcalHandler know when the __dbg is finished and they flush their buffers.

In theory, Linux fflush is not a syscall because it just flushes "user-space" buffers. As we do not have this difference, we are still going to call fflush as a syscall.

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[codspeed-hq]

CodSpeed Performance Report

Merging #7126 will not alter performance

_{Comparing xunilrj/dbg-fflush (6908f91) with master (d258921)}

Summary

✅ 22 untouched benchmarks

[tritao]

Can you maybe explain the rationale for this? Is there a reason the output cannot be flushed as it arrives? Maybe due to performance reasons on the consumer side?

[xunilrj]

It can. The hardest part is to know when '__dbg' stops printing.

This will allow 'EcalHandlers' to print the whole thing as just one string when needed.