Fix detection of genkernel initramfs for xen

https://bugs.gentoo.org/show_bug.cgi?id=463992

--- a/util/grub.d/20_linux_xen.in
+++ b/util/grub.d/20_linux_xen.in
@@ -174,6 +174,7 @@
 
 title_correction_code=
 
+machine=`uname -m`
 case "$machine" in
     i?86) GENKERNEL_ARCH="x86" ;;
     mips|mips64) GENKERNEL_ARCH="mips" ;;