refactor(genimage): take the debootstrap architecture from the shared mapping

genimage translated one architecture for debootstrap, x86_64 to amd64, and
compared against a bareword rather than a string, which only resolves because
the script does not enable strict subs.

Read the name from xCAT::Utils, which genimage already loads. Every
architecture reaches debootstrap with the name it does today.

xCAT-server/share/xcat/netboot/ubuntu/genimage

@@ -215,8 +215,7 @@ foreach (@ndrivers, @default_ndrivers) {
     }
 }
 
-my $uarch = $arch;
-$uarch = "amd64" if ($arch eq x86_64);
+my $uarch = xCAT::Utils->debian_arch($arch);
 
 
 unless ($onlyinitrd) {