UBUNTU: SAUCE: fix FTBFS in machine_real_restart()

UBUNTU: SAUCE: fix FTBFS in machine_real_restart()

(focal-amd64)ppisati@kathleen:~/linux$ make -j128
  DESCEND  objtool
  CALL    scripts/atomic/check-atomics.sh
  CALL    scripts/checksyscalls.sh
  CHK     include/generated/compile.h
  CC      arch/x86/kernel/reboot.o
  CHK     kernel/kheaders_data.tar.xz
arch/x86/kernel/reboot.o: warning: objtool: machine_real_restart()+0x63: retpoline_safe hint not an indirect jump/call
make[2]: *** [scripts/Makefile.build:275: arch/x86/kernel/reboot.o] Error 255
make[2]: *** Deleting file 'arch/x86/kernel/reboot.o'
make[1]: *** [scripts/Makefile.build:496: arch/x86/kernel] Error 2
make: *** [Makefile:1734: arch/x86] Error 2
make: *** Waiting for unfinished jobs....

Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>