Fix possible ODR violation (#25368)

Fix possible ODR violation (this is what clang says when building with
C++20 modules and doesn't allow compilation to proceed further)

Author: j4niwzis

system/include/emscripten/em_asm.h

@@ -193,12 +193,9 @@ struct __em_asm_sig_builder {};
 
 template<typename... Args>
 struct __em_asm_sig_builder<__em_asm_type_tuple<Args...> > {
-  static const char buffer[sizeof...(Args) + 1];
+  inline static const char buffer[sizeof...(Args) + 1] = { __em_asm_sig<Args>::value..., 0 };
 };
 
-template<typename... Args>
-const char __em_asm_sig_builder<__em_asm_type_tuple<Args...> >::buffer[] = { __em_asm_sig<Args>::value..., 0 };
-
 // We move to type level with decltype(make_tuple(...)) to avoid double
 // evaluation of arguments. Use __typeof__ instead of decltype, though,
 // because the header should be able to compile with clang's -std=c++03.