Rename _end to end

This matches the libc more closely.

src/arch/amd64/uKernel.ld

@@ -52,7 +52,7 @@ SECTIONS
 		. = ALIGN(4096);
 	}
 
-	_end = .;
+	end = .;
 
 	/DISCARD/ :
 	{

src/arch/i386/uKernel.ld

@@ -44,7 +44,7 @@ SECTIONS
 		*(.bss)
 	}
 
-	_end = .;
+	end = .;
 
 	/* The compiler may produce other sections, by default it will put them in
 	   a segment with the same name. Simply add stuff here as needed. */