builder: use the @ syntax to reduce command line length if needed

This is required if the number of object files is too high.

Fix arduino#839

Author: cmaglie

Diff for: legacy/builder/phases/linker.go

@@ -60,6 +60,14 @@ func link(ctx *types.Context, objectFiles paths.PathList, coreDotARelPath *paths
 	quotedObjectFiles := utils.Map(objectFiles.AsStrings(), wrapWithDoubleQuotes)
 	objectFileList := strings.Join(quotedObjectFiles, " ")
 
+	// If the number of object files is too big, write it down into a file and reference
+	// it using the @ parameter "@path/to/object_files.txt".
+	if len(objectFileList) > 500 {
+		objectFilesTxt := ctx.BuildPath.Join("object_files.txt")
+		objectFilesTxt.WriteFile([]byte(strings.Join(quotedObjectFiles, "\n") + "\n"))
+		objectFileList = "@" + objectFilesTxt.String()
+	}
+
 	properties := buildProperties.Clone()
 	properties.Set(constants.BUILD_PROPERTIES_COMPILER_C_ELF_FLAGS, properties.Get(constants.BUILD_PROPERTIES_COMPILER_C_ELF_FLAGS))
 	properties.Set(constants.BUILD_PROPERTIES_COMPILER_WARNING_FLAGS, properties.Get(constants.BUILD_PROPERTIES_COMPILER_WARNING_FLAGS+"."+ctx.WarningsLevel))