Add a stub of __context__ builtin from kernel's sparse

[jprotopopov-ut]

Adds a stub for __context__ builtin of Linux kernel Sparse semantic checker.

https://sparse.docs.kernel.org/en/latest/annotations.html#context-ctxt-entry-exit

[sim642]

Is it really necessary to replace it with an arbitrary constant 1?
Why does it need to be handled in CIL? Couldn't it just remain as a normal function call that's added to Goblint's LibraryFunctions?

Or alternatively, wouldn't it be enough to introduce initSparseBuiltins like

cil/src/cil.ml

Line 2903 in e21285a

let initGccBuiltins () : unit =

if it needs an implicit declaration and a signature?
Most builtins are not specifically handled here, only the ones that really have to do something at compile time. But this __context__ doesn't seem to require anything like this.

[jprotopopov-ut]

If I remember correctly, this built-in appeared in rather inconvenient places (e.g. function declarations), but at the moment I cannot find suitable examples. Once the other pull requests are dealt with, I will try to convert it into a library function to see whether it suffices. For now, converting into a draft.

[sim642]

If I remember correctly, this built-in appeared in rather inconvenient places (e.g. function declarations), but at the moment I cannot find suitable examples.

From what I gathered, there's two flavors of this (https://elixir.bootlin.com/linux/v6.19-rc5/source/include/linux/compiler_types.h#L55-L61):

1. Function declaration attributes. CIL should have no problem with these being present: they should be parsed by the general __attribute__ logic and just carried around.
2. The __context__ "function" calls within function bodies. I guess __context__ is never declared/defined as anything but just supposed to directly stay around for Sparse. I suppose CIL might complain that __context__ is an undeclared identifier, in which case the builtins list might just help.