Always use LF line endings in generated stdlib code

The code generation uses a mixture of literal `\n` characters and `appendln`. The latter insert `\r\n` on Windows by default, causing generated files to contain a mixture of line endings.

This commit sets the `line.separator` system property for the generator to `\n` so that `appendln` will never insert `\r` characters. As an additional measure, `.gitattributes` files were added to checkout generated stdlib files always with LF line endings.

libraries/stdlib/common/src/generated/.gitattributes

@@ -0,0 +1 @@
+*.kt eol=lf

libraries/stdlib/js-ir/src/generated/.gitattributes

@@ -0,0 +1 @@
+*.kt eol=lf

libraries/stdlib/js/src/generated/.gitattributes

@@ -0,0 +1 @@
+*.kt eol=lf

libraries/stdlib/jvm/src/generated/.gitattributes

@@ -0,0 +1 @@
+*.kt eol=lf

libraries/tools/kotlin-stdlib-gen/build.gradle

@@ -33,4 +33,5 @@ task run(type: JavaExec) {
     main 'generators.GenerateStandardLibKt'
     classpath sourceSets.main.runtimeClasspath
     args = ["${rootDir}"]
+    systemProperty 'line.separator', '\n'
 }