Mangle "main" as "__main_void" on wasm32-wasi

On wasm, the age-old C trick of having a main function which can either have
no arguments or argc+argv doesn't work, because wasm requires caller and
callee signatures to match. WASI's current strategy is to have compilers
mangle main's name to indicate which signature they're using. Rust uses the
no-argument form, which should be mangled as `__main_void`.

This is needed on wasm32-wasi as of #105395.

compiler/rustc_target/src/spec/wasm32_wasi.rs

@@ -104,6 +104,10 @@ pub fn target() -> Target {
     // `args::args()` makes the WASI API calls itself.
     options.main_needs_argc_argv = false;
 
+    // And, WASI mangles the name of "main" to distinguish between different
+    // signatures.
+    options.entry_name = "__main_void".into();
+
     Target {
         llvm_target: "wasm32-wasi".into(),
         pointer_width: 32,