vfork: do not use hard coded 4 for performing syscalls

[badochov]

JIRA: RTOS-697

Description

Motivation and Context

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

How Has This Been Tested?

• Already covered by automatic testing.
• New test added: (add PR link here).
• Tested by hand on: (list targets here).

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing linter checks and tests passed.
• My changes generate no new compilation warnings for any of the targets.

Special treatment

• This PR needs additional PRs to work (list the PRs, preferably in merge-order).
• I will merge this PR by myself when appropriate.

[badochov]

kudos to @gerard5 pointing me to .equ directive

[github-actions]

Unit Test Results

6 002 tests  ±0   5 368 ✔️ ±0   29m 14s ⏱️ - 1m 23s
   346 suites ±0      634 💤 ±0 
       1 files   ±0          0 ❌ ±0 

Results for commit ad3048d. ± Comparison against base commit ae1c008.