runtime: add runtime.fnctl link directives

[leongross]

Building u-root commands with tinygo (see u-root/u-root#2979) returns linking errors for the system runtime function runtime.fcntl. This PR intends to fix this issue. Affected u-root cmdlets:

• localboot
• netbootxyz
• newsshd
• systemboot

[aykevl]

Why are you trying to do a direct system call in syscall_libc.go? Syscall6 and the like are only supported on Windows and Linux, not on many other systems.

What about calling the libc function fcntl instead?

[leongross]

Creating a wrapper to the libc sounds like a good idea.

[aykevl]

This is a bad idea. It should not just ignore the call and return without an error.
Is this intended to call syscall_fcntl perhaps?

And then, any reason not to call the libc function instead (like with syscall_Getpagesize for example).

[aykevl]

I think you meant runtime.fcntl?

Suggested change

	//go:linkname syscall_fcntl runtime/runtime.fcntl
	//go:linkname syscall_fcntl runtime.fcntl

[leongross]

I am perplexed by the linker's behavior. After reconsidering your design choices, what I want to do is the following:

• create function Fcntl in syscall_unix and syscall_libc
• in os_linux add the function declaration for fcntl
• link the syscall function over the runtime.fcntl function (since we cannot import the syscall package due to circular dependencies)

I tried:

• link the runtime/fcnlt function to syscall.Fcntl -> did not work
• link the syscall.Fcntl function(s) to runtime.fcntl -> did not work
• declare runtime.fcntl as linkable (//go:linkname fcntl) and link from the syscall package up -> did not work

I realized it only worked when I defined runtime.fcntl to return nothing. I would really appreciate your input on this.