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.

compiler/rustc_session/src/options.rs

@@ -279,6 +279,7 @@ macro_rules! options {
         pub const parse_tls_model: &str =
             "one of supported TLS models (`rustc --print tls-models`)";
         pub const parse_target_feature: &str = parse_string;
+        pub const parse_wasi_exec_model: &str = "either `command` or `reactor`";
     }
 
     #[allow(dead_code)]
@@ -722,6 +723,15 @@ macro_rules! options {
                 None => false,
             }
         }
+
+        fn parse_wasi_exec_model(slot: &mut Option<WasiExecModel>, v: Option<&str>) -> bool {
+            match v {
+                Some("command")  => *slot = Some(WasiExecModel::Command),
+                Some("reactor") => *slot = Some(WasiExecModel::Reactor),
+                _ => return false,
+            }
+            true
+        }
     }
 ) }
 
@@ -1166,9 +1176,17 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
         "in general, enable more debug printouts (default: no)"),
     verify_llvm_ir: bool = (false, parse_bool, [TRACKED],
         "verify LLVM IR (default: no)"),
+    wasi_exec_model: Option<WasiExecModel> = (None, parse_wasi_exec_model, [TRACKED],
+        "whether to build a wasi command or reactor"),
 
     // This list is in alphabetical order.
     //
     // If you add a new option, please update:
-    // - src/librustc_interface/tests.rs
+    // - compiler/rustc_interface/src/tests.rs
+}
+
+#[derive(Clone, Hash)]
+pub enum WasiExecModel {
+    Command,
+    Reactor,
 }