Force forward slash for writing @..txt files for gcc

It seems that on windows the compiler is not able to handle files
written in the format

  "C:\path\to\object.o"

but it must be either

  "C:/path/to/object.o"

or

  "C:\\path\\to\\object.o"

I don't know if this is a quirk of gcc or it has something to do with
golang doing some extra excaping on command.Exec that we must do also on
@..txt files.

Author: cmaglie

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

@@ -16,6 +16,7 @@
 package phases
 
 import (
+	"path/filepath"
 	"strings"
 
 	"github.com/arduino/arduino-cli/legacy/builder/builder_utils"
@@ -57,14 +58,17 @@ func (s *Linker) Run(ctx *types.Context) error {
 }
 
 func link(ctx *types.Context, objectFiles paths.PathList, coreDotARelPath *paths.Path, coreArchiveFilePath *paths.Path, buildProperties *properties.Map) error {
-	quotedObjectFiles := utils.Map(objectFiles.AsStrings(), wrapWithDoubleQuotes)
-	objectFileList := strings.Join(quotedObjectFiles, " ")
+	objectFileList := strings.Join(utils.Map(objectFiles.AsStrings(), wrapWithDoubleQuotes), " ")
 
 	// 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"))
+		// The files should be written with forward slashes, otherwise on Windows
+		// gcc seems to be not able to find the file... does golang do extra escaping
+		// on the command line that we do not do?
+		files := utils.Map(utils.Map(objectFiles.AsStrings(), filepath.ToSlash), wrapWithDoubleQuotes)
+		objectFilesTxt.WriteFile([]byte(strings.Join(files, "\n") + "\n"))
 		objectFileList = "@" + objectFilesTxt.String()
 	}