build: specify minimum versions of gcc and binutils

To help people avoid having to figure out what versions of gcc and
binutils need to be supported document them explicitly.

Signed-off-by: Doug Goldstein <cardoe@cardoe.com>
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

README

@@ -35,9 +35,13 @@ Second, there are a number of prerequisites for building a Xen source
 release. Make sure you have all the following installed, either by
 visiting the project webpage or installing a pre-built package
 provided by your OS distributor:
-    * GCC v4.1 or later
+    * GCC
+      - For x86 4.1.2_20070115 or later
+      - For ARM 4.8 or later
     * GNU Make
-    * GNU Binutils
+    * GNU Binutils:
+      - For x86 2.16.91.0.5 or later
+      - For ARM 2.24 or later
     * Development install of zlib (e.g., zlib-dev)
     * Development install of Python v2.3 or later (e.g., python-dev)
     * Development install of curses (e.g., libncurses-dev)