[wasm] tool/m4/ruby_wasm_tools.m4: Add default value for OBJCOPY

The tool is used to build shared libraries but system installed
tools usually don't support WebAssembly, so use WASI SDK's tools by default.

tool/m4/ruby_wasm_tools.m4

@@ -19,6 +19,7 @@ AC_DEFUN([RUBY_WASM_TOOLS],
         LD="${LD:-${WASI_SDK_PATH}/bin/clang}"
         AR="${AR:-${WASI_SDK_PATH}/bin/llvm-ar}"
         RANLIB="${RANLIB:-${WASI_SDK_PATH}/bin/llvm-ranlib}"
+        OBJCOPY="${OBJCOPY:-${WASI_SDK_PATH}/bin/llvm-objcopy}"
     ])
 ])
 ])dnl