make installers: give MSVC setup "x86_amd64" instead of "x64" by default

The "x86_amd64" mode seems to work more often, especially with
Visual Studio Express installations.

original commit: 9c05eff8751cc104f430b8ed231268e79ecf5303

pkgs/distro-build-pkgs/distro-build-client/doc.txt

@@ -216,8 +216,8 @@ Site-configuration keywords (where <string*> means no spaces, etc.):
   #:bits <integer> --- 32 or 64, affects Visual Studio mode
 
   #:vc <string*> --- provided to "vcvarsall/bat" to select the Visual
-    Studio build mode; the default is "x86" or "x64", depending on
-    `#:bits'
+    Studio build mode; the default is "x86" or "x86_amd64", depending
+    on `#:bits'
 
   #:sign-identity <string> --- provides an identity to be passed to
     `codesign` for code signing on Mac OS X (for all executables in a

pkgs/distro-build-pkgs/distro-build-server/drive-clients.rkt

@@ -299,7 +299,7 @@
   (define vc (or (get-opt c '#:vc)
                  (if (= bits 32)
                      "x86"
-                     "x64")))
+                     "x86_amd64")))
   (define j (or (get-opt c '#:j) 1))
   (define (cmd . args) 
     (list "cmd" "/c" (shell-protect (apply ~a args) platform)))