Cache Hardware.is_64_bit?

IO#popen is called for each call to Hardware.is_64_bit?; this becomes
costly when it is repeatedly invoked. Luckily it is an invariant, so we
can store it in a class variable.

False is a valid value for this method, so the usual ||= idiom is not
applicable.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/hardware.rb

@@ -55,7 +55,8 @@ class Hardware
   end
 
   def self.is_64_bit?
-    self.sysctl_bool("hw.cpu64bit_capable")
+    return @@is_64_bit if defined? @@is_64_bit
+    @@is_64_bit = self.sysctl_bool("hw.cpu64bit_capable")
   end
   
   def self.bits