Tracking issue for `linux_syscall`

[newpavlov]

PR: #63745

Unresolved questions:

• Should we add syscalls to core or keep only in std?
• Do we still need the workaround for syscall6 on x86?

[hellow554]

How is a syscall supported in a core environment? I don't think that that will make sense

[newpavlov]

In the essence it's not that different from supporting target_arch which we do in core::arch::x86_64, but this time those functions will be feature-gated behind target_os. And since calling syscalls involves just a tiny assembly snippet, it would not cause core bloat. Read the discussion linked in the PR. Also I think this issue is relevant as well: #36193

[joshtriplett]

@newpavlov I would very much like to use these functions in no_std projects that don't link to std or libc. That would be my primary use case for direct use of syscalls.