runtime: add runtime.fcntl link directives

[leongross]

Building u-root commands with tinygo (see https://github.com/u-root/u-root/issues/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

[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.