Add support for resettable protection domains

[nspin]

This PR adds support for the creation of Microkit protection domains which are capable of restoring themselves to their initial state from code and data in read-only memory. Resettable protection domains may reset themselves with the new sel4_reset::reset() function (e.g. in a panic handler), or a parent protection domain may reset a child protection domain by setting its PC back to its entrypoint.

To illustrate how all of this works, here are the steps for creating such a resettable protection domain:

• Link it against the sel4-reset crate, using, for example, extern se4_reset. This crate defines the _reset symbol, which will serve as the protection domain's new entrypoint. _reset resets all writable segments to their initial state from data in read-only segments, with the exception of the footprint of the .persistent ELF section, and then passes control to _start.
• Use one of the new --target *-sel4-microkit-resettable*.json target specs, which augments the base Microkit target specs with a linker script fragment that designates _reset as the ELF's entrypoint and gathers and places the .persistent ELF section.
• Process the program image with the sel4-reset-cli program, which adds a read-only segment containing all of the data needed to restore the writable segments to their initial state.

See crates/private/tests/microkit/reset/src/bin/test.rs for all of this in action.

[nspin]

Uses seL4/microkit#164

[podhrmic]

Very cool feature!