Improve support for program reset

[nspin]

#167 added support for resettable protection domains.

This PR generalizes this to support for resettable programs in general. This PR also improves upon previous reset support by reducing how invasive the sel4-reset crate is. For example, after this PR, *-resettable.json target specs are not necessary.

This PR also introduces a new, simpler way of patching ELF files. Before this PR, all ELF patching in this project used the sel4-synthetic-elf crate which creates a new ELF file including the data of the original ELF file plus new data. This PR introduces a new approach, where the program to be patched ensures the presence of some extra program header slots (via elfnotes), and the patching tool just modifies the ELF file by replacing those spare program header slots with new program headers pointing to new segments, whose data is appended to the original file. This new approach is much simpler than the old approach. The only downside of the new approach is that it makes assumptions about how the linker handles elfnotes.

This PR also adds a test for sel4-reset using the new minimal Linux runtime.

This PR moves sel4-reset out of ./crates/experimental up into ./crates.