Make foreign context kernel procedures static

[Fumuran]

What should be done?

Original comment: #890 (comment).
start_foreign_context_offset and end_foreign_context_offset kernel procedures should be reworked to become static instead of being executed dynamically.

How should it be done?

Offsets of the start_foreign_context_offset and end_foreign_context_offset procedures should be removed to make it possible to execute them only using syscall.<proc_name> (instead of syscall.exec_kernel_proc).

When is this task done?

The task is done when it will be possible to invoke these procedures only by their names.

Additional context

No response