gdb: Do not print empty-group regs when printing general ones

gdb: Do not print empty-group regs when printing general ones

When the command "info registers" (same as "info registers general"),
is issued, _all_ the registers from a tdesc XML are printed. This
includes the registers with empty register groups (set as "") which
are supposed to be only printed by "info registers all" (or "info
all-registers").

This bug got introduced after all the overhauls that the
tdesc_register_in_reggroup_p() went through. You can see that the
logic of tdesc_register_in_reggroup_p() did NOT remain the same after
all those changes:

  git difftool c9c895b9666..HEAD -- gdb/target-descriptions.c

With the current implementation, when the reg->group is an empty
string, this function returns -1, while in the working revision
(c9c895b9666), it returned 0. This patch makes sure that the 0 is
returned again.

The old implementation of tdesc_register_in_reggroup_p() returned
-1 when "reggroup" was set to "all_reggroups" at line 4 below:

1  tdesc_register_reggroup_p (...)
2  {
3   ...
4   ret = tdesc_register_in_reggroup_p (gdbarch, regno, reggroup);
5   if (ret != -1)
6     return ret;
7
8   return default_register_reggroup_p (gdbarch, regno, reggroup);
9  }

As a result, the execution continued at line 8 and the
default_register_reggroup_p(..., reggroup=all_reggroups) would
return 1. However, with the current implementation of
tdesc_register_in_reggroup_p() that allows checking against any
arbitrary group name, it returns 0 when comparing the "reg->group"
against the string "all" which is the group name for "all_reggroups".
I have added a special check to cover this case and
"info all-registers" works as expected.

gdb/ChangeLog:

	* target-descriptions.c (tdesc_register_in_reggroup_p): Return 0
	when reg->group is empty and reggroup is not.

Change-Id: I9eaf9d7fb36410ed5684ae652fe4756b1b2e61a3