Runtime: define get_return_address for WebAssembly

Author: MaxDesiatov

stdlib/public/runtime/Exclusivity.cpp

@@ -38,6 +38,8 @@
 #elif _MSC_VER
 #include <intrin.h>
 #define get_return_address() _ReturnAddress()
+#elif defined(__wasi__)
+#define get_return_address() ((void*) 0)
 #else
 #error missing implementation for get_return_address
 #define get_return_address() ((void*) 0)