proper jvm_platform, notably for org.sqlite.lib.path;

--- a/src/Pure/System/platform.scala	Thu May 06 23:20:02 2021 +0200
+++ b/src/Pure/System/platform.scala	Thu May 06 23:28:30 2021 +0200
@@ -64,6 +64,7 @@
 
   def os_name: String =
     family match {
+      case Family.linux_arm => "linux"
       case Family.macos => "darwin"
       case _ => family.toString
     }