Don't run static constructors on arbitrary user exports.

Previously, "new-style commmands" considered every user-defined
export to be a potential command entrypoint, so wasi-libc and wasm-ld
cooperated to run the user's static constructors on each entrypoint.

This form of new-style command turned out not to be useful, and it
interferes with some use cases, so disable it.

This is done by making an explicit call to `__wasm_call_ctors`, which
tells wasm-ld that it shouldn't synthesize any calls to
`__wasm_call_ctors` on its own.

libc-bottom-half/crt/crt1-command.c

@@ -1,18 +1,24 @@
 #include <wasi/api.h>
-#include <stdlib.h>
 extern void __wasm_call_ctors(void);
 extern int __main_void(void);
 extern void __wasm_call_dtors(void);
 
 __attribute__((export_name("_start")))
 void _start(void) {
+    // The linker synthesizes this to call constructors.
+    __wasm_call_ctors();
+
     // Call `__main_void` which will either be the application's zero-argument
     // `__main_void` function or a libc routine which obtains the command-line
     // arguments and calls `__main_argv_argc`.
     int r = __main_void();
 
-    // If main exited successfully, just return, otherwise call `exit`.
+    // Call atexit functions, destructors, stdio cleanup, etc.
+    __wasm_call_dtors();
+
+    // If main exited successfully, just return, otherwise call
+    // `__wasi_proc_exit`.
     if (r != 0) {
-        exit(r);
+        __wasi_proc_exit(r);
     }
 }

libc-bottom-half/crt/crt1.c

@@ -1,24 +1,3 @@
-#include <wasi/api.h>
-extern void __wasm_call_ctors(void);
-extern int __main_void(void);
-extern void __wasm_call_dtors(void);
-
-__attribute__((export_name("_start")))
-void _start(void) {
-    // The linker synthesizes this to call constructors.
-    __wasm_call_ctors();
-
-    // Call `__main_void` which will either be the application's zero-argument
-    // `__main_void` function or a libc routine which obtains the command-line
-    // arguments and calls `__main_argv_argc`.
-    int r = __main_void();
-
-    // Call atexit functions, destructors, stdio cleanup, etc.
-    __wasm_call_dtors();
-
-    // If main exited successfully, just return, otherwise call
-    // `__wasi_proc_exit`.
-    if (r != 0) {
-        __wasi_proc_exit(r);
-    }
-}
+// We compile a plain crt1.o for toolchain compatibility, but it's
+// identical to crt1-command.o.
+#include <crt1-command.c>