Add a check to reactor modules to ensure _initialize is only called once (#388)

Calling _initialize multiple times is undefined behavior, since the
ctors are not guaranteed to be idempotent. We should have this safety
check which is similar to #329.

Author: TerrorJack

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

@@ -1,7 +1,27 @@
+#if defined(_REENTRANT)
+#include <stdatomic.h>
+extern void __wasi_init_tp(void);
+#endif
 extern void __wasm_call_ctors(void);
 
 __attribute__((export_name("_initialize")))
 void _initialize(void) {
+#if defined(_REENTRANT)
+    static volatile atomic_int initialized = 0;
+    int expected = 0;
+    if (!atomic_compare_exchange_strong(&initialized, &expected, 1)) {
+        __builtin_trap();
+    }
+
+    __wasi_init_tp();
+#else
+    static volatile int initialized = 0;
+    if (initialized != 0) {
+        __builtin_trap();
+    }
+    initialized = 1;
+#endif
+
     // The linker synthesizes this to call constructors.
     __wasm_call_ctors();
 }