xen: fix unspecified behavior in tr invocation

The result of the command is undefined according to the specification if
the "string2" argument in tr is shorter than "string1". GNU tr behaves
correctly by extending "string2" to repeat the last character.

Fixes: eb61a4fb14 ("xen: fix header guard generation for asm-generic headers")
Reported-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Nicola Vetrini <nicola.vetrini@bugseng.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/scripts/Makefile.asm-generic

@@ -34,7 +34,7 @@ unwanted    := $(filter-out $(generic-y) $(generated-y),$(old-headers))
 quiet_cmd_wrap = WRAP    $@
       cmd_wrap = \
 	arch=$$(echo $@ | sed -n 's:.*arch/\([^/]*\)/.*:\1:p' | tr a-z A-Z); \
-	upper=$$(echo $*.h | tr a-z A-Z | tr '/.-' '__'); \
+	upper=$$(echo $*.h | tr a-z A-Z | tr '/.-' '___'); \
 	echo "\#ifndef $${arch}_GENERIC_$${upper}" > $@.new; \
 	echo "\#define $${arch}_GENERIC_$${upper}" >> $@.new; \
 	echo "\#include <asm-generic/$*.h>" >> $@.new; \