Rollup merge of #79997 - coolreader18:wasm-reactor, r=alexcrichton

Emit a reactor for cdylib target on wasi

Fixes #79199, and relevant to #73432

Implements wasi reactors, as described in WebAssembly/WASI#13 and [`design/application-abi.md`](https://github.com/WebAssembly/WASI/blob/master/design/application-abi.md)

Empty `lib.rs`, `lib.crate-type = ["cdylib"]`:

```shell
$ cargo +reactor build --release --target wasm32-wasi
   Compiling wasm-reactor v0.1.0 (/home/coolreader18/wasm-reactor)
    Finished release [optimized] target(s) in 0.08s
$ wasm-dis target/wasm32-wasi/release/wasm_reactor.wasm >reactor.wat
```
`reactor.wat`:
```wat
(module
 (type $none_=>_none (func))
 (type $i32_=>_none (func (param i32)))
 (type $i32_i32_=>_i32 (func (param i32 i32) (result i32)))
 (type $i32_=>_i32 (func (param i32) (result i32)))
 (type $i32_i32_i32_=>_i32 (func (param i32 i32 i32) (result i32)))
 (import "wasi_snapshot_preview1" "fd_prestat_get" (func $__wasi_fd_prestat_get (param i32 i32) (result i32)))
 (import "wasi_snapshot_preview1" "fd_prestat_dir_name" (func $__wasi_fd_prestat_dir_name (param i32 i32 i32) (result i32)))
 (import "wasi_snapshot_preview1" "proc_exit" (func $__wasi_proc_exit (param i32)))
 (import "wasi_snapshot_preview1" "environ_sizes_get" (func $__wasi_environ_sizes_get (param i32 i32) (result i32)))
 (import "wasi_snapshot_preview1" "environ_get" (func $__wasi_environ_get (param i32 i32) (result i32)))
 (memory $0 17)
 (table $0 1 1 funcref)
 (global $global$0 (mut i32) (i32.const 1048576))
 (global $global$1 i32 (i32.const 1049096))
 (global $global$2 i32 (i32.const 1049096))
 (export "memory" (memory $0))
 (export "_initialize" (func $_initialize))
 (export "__data_end" (global $global$1))
 (export "__heap_base" (global $global$2))
 (func $__wasm_call_ctors
  (call $__wasilibc_initialize_environ_eagerly)
  (call $__wasilibc_populate_preopens)
 )
 (func $_initialize
  (call $__wasm_call_ctors)
 )
 (func $malloc (param $0 i32) (result i32)
  (call $dlmalloc
   (local.get $0)
  )
 )
 ;; lots of dlmalloc, memset/memcpy, & libpreopen code
)
```

I went with repurposing cdylib because I figured that it doesn't make much sense to have a wasi shared library that can't be initialized, and even if someone was using it adding an `_initialize` export is a very small change.

1 files changed, 2 insertions, 1 deletions

diff --git a/compiler/rustc_interface/src/tests.rs b/compiler/rustc_interface/src/tests.rs
index 3e94f163773..55d521a9b5f 100644
--- a/compiler/rustc_interface/src/tests.rs
+++ b/compiler/rustc_interface/src/tests.rs
@@ -7,7 +7,7 @@ use rustc_session::config::{build_configuration, build_session_options, to_crate
 use rustc_session::config::{rustc_optgroups, ErrorOutputType, ExternLocation, Options, Passes};
 use rustc_session::config::{CFGuard, ExternEntry, LinkerPluginLto, LtoCli, SwitchWithOptPath};
 use rustc_session::config::{
-    Externs, OutputType, OutputTypes, SanitizerSet, SymbolManglingVersion,
+    Externs, OutputType, OutputTypes, SanitizerSet, SymbolManglingVersion, WasiExecModel,
 };
 use rustc_session::lint::Level;
 use rustc_session::search_paths::SearchPath;
@@ -597,6 +597,7 @@ fn test_debugging_options_tracking_hash() {
     tracked!(unleash_the_miri_inside_of_you, true);
     tracked!(use_ctors_section, Some(true));
     tracked!(verify_llvm_ir, true);
+    tracked!(wasi_exec_model, Some(WasiExecModel::Reactor));
 }
 
 #[test]