Re-instate this as an arch-independent package, and remove gcc call

We're now assuming that the size of size_t is a long unsigned integer.
In the kernel there's a comment:

/*
 * Most 32 bit architectures use "unsigned int" size_t,
 * and all 64 bit architectures use "unsigned long" size_t.
 */

On 32 bit architectures, an unsigned int is the same size as an unsigned
long anyway, so I think this assumption is OK.

lib/linux/c_struct.rb

@@ -82,8 +82,8 @@ class CStruct
   begin
     require 'linux/c_struct_sizeof_size_t.rb'
   rescue LoadError
-    warn "Falling back to gcc to determine sizeof size_t." if $VERBOSE
+    warn "netlinkrb: Assuming size_t is a long unsigned int." if $DEBUG
-    SIZEOF_SIZE_T = Integer(`echo __SIZEOF_SIZE_T__ | gcc -E -P -`) rescue 1.size
+    SIZEOF_SIZE_T = [0].pack("L_").size
   end
 
   define_type :size_t,