vlib: use builtin flush functions

[ttytm]

🤖[deprecated] Generated by Copilot at 0d8289f

Refactored some builtin and os functions to use flush_stdout and flush_stderr from vlib/builtin/builtin.c.v. This improves code consistency and maintainability across different modules and platforms.

🤖[deprecated] Generated by Copilot at 0d8289f

• Refactor the code to use flush_stdout and flush_stderr functions from vlib/builtin/builtin.c.v instead of calling C functions directly (link, link, link, link, link, link)
• Move the code for printing to standard output and error from vlib/os/os.c.v to vlib/builtin/builtin.c.v as part of a larger restructuring of the os module (link)

[spytheman]

Good work. I had that in my TODO list for several weeks :-| .