Add -Zjit-mode flag

[bjorn3]

This flag tells the codegen backend to immediately execute the code it builds. This is exactly what miri needs as it is unable to produce real executables. Adding a builtin flag would allow cargo to add native support for this without miri needing to do a bunch of hacks in its cargo wrapper. Furthermore cg_clif also has a jit mode that benefits from it.

[rustbot]

The Miri subtree was changed

cc @rust-lang/miri

[rustbot]

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[Urgau]

This is kind-of a nit, but the naming of the flag implies to me that we generate a binary that calls back rustc at runtime to generate the machine code where necessary. What about -Zrun instead? or -Zexecute?

[bjorn3]

What about -Zrun instead? or -Zexecute?

I like those names. I don't have much of a preference which of the two to use.