opam: retarget the eio, solo5 and unikraft pins
eio moves to a dev commit: it adds portable Net socket options, so a
consumer can set TCP_NODELAY and friends without linking eio.unix. Every package
that depends on eio carries the pin, since opam resolves pin-depends per package
and not from the root.
The solo5 and unikraft cross-toolchains move to wip-macos+runtime_events+lto,
one branch carrying macOS cross-build support, an in-process runtime_events ring
and a flambda compiler that defaults to link-time DCE. The unikernel arms link
with -use-lto, so the compiler has to carry those patches.