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.
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