[kernel] Update default machdep to x86_64 or FRAMAC_MACHDEP

man/frama-c.1

@@ -395,7 +395,9 @@ uses \f[I]machine\f[R] as the current machine-dependent configuration
 (size of the various integer types, endiandness, \&...).
 The list of currently supported machines is available through option
 \f[I]-machdep help\f[R].
-Default is \f[B]x86_32\f[R].
+Default is \f[B]x86_64\f[R].
+The environment variable FRAMAC_MACHDEP can be used to override the default
+value. The command line parameter still has priority over the default value.
 .TP
 .B -main \f[I]f\f[R]
 sets \f[I]f\f[R] as the entry point of the analysis.

src/kernel_services/plugin_entry_points/kernel.ml

@@ -931,11 +931,15 @@ module Machdep =
     (struct
       let module_name = "Machdep"
       let option_name = "-machdep"
-      let default = "x86_32"
+      let default =
+        try Sys.getenv "FRAMAC_MACHDEP"
+        with Not_found -> "x86_64"
       let arg_name = "machine"
       let help =
         "use <machine> as the current machine dependent configuration. \
-         See \"-machdep help\" for a list"
+         See \"-machdep help\" for a list. The environment variable \
+         FRAMAC_MACHDEP can be used to override the default value. The command \
+         line parameter still has priority over the default value"
     end)
 
 let () = Parameter_customize.set_group parsing