xtensa: Properly strdup string when building reggroup

xtensa: Properly strdup string when building reggroup

I noticed this while looking at the reggroup intializations.  It seems
for xtensa the "cpN" reggroup->name is getting assigned to the same text
pointer for each iteration of XTENSA_MAX_COPROCESSOR.

Note, internally reggroup_new() does not do any xstrdup().

gdb/ChangeLog:
2017-08-15  Stafford Horne  <shorne@gmail.com>

	* xtensa-tdep.c (xtensa_init_reggroups): Use xstrdup for cpname.