EII, support `extern_item_impls`

[boozook]

Please support "lazy linking", rust feature extern_item_impls (or eii).

Currently Miri throws error on try to call "eii-implementation":

error: unsupported operation: can't call foreign function `_RNvCs2.._event_handler13event_handler` on OS `none`
  --> sdk/entry/src/lib.rs:84:4
   |
84 |             self::event_handler(api, event, arg)
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ unsupported operation occurred here
   |
   = help: this means the program tried to do something Miri does not support; it does not indicate a bug in the program

Steps to reproduce

#![no_std]
#![feature(extern_item_impls)]

#[eii(entry)]
fn foo();

// isolate in mod or another crate:
mod __ {
	#[super::entry]
	fn foo() {}
}


#[cfg(miri)]
#[unsafe(no_mangle)]
fn miri_start(_argc: isize, _argv: *const *const u8) -> isize {
	foo(); // <— unsupported operation occurred here
	0
}

[oli-obk]

cc @jdonszelmann

[RalfJung]

Part of rust-lang/rust#125418

[bjorn3]

Once rust-lang/rust#159843 lands, EIIs that don't have a default impl should work without requiring any extra work on the miri side. Those that do have a default impl will still need extra work to be handled as weak symbols.

[RalfJung]

Miri supports weak symbols, but not "weak symbol aliases", whatever those are.

[bjorn3]

A symbol alias is a symbol which points to the same location as another symbol (aka it aliases the other symbol). A weak symbol alias is a symbol alias that is a weak definition, so another symbol or symbol alias can override it. EII defaults are implemented by having a weak symbol alias from the EII to the default impl and then a possibly strong symbol alias for a definition.