gdb: move struct reggroup into reggroups.h header

Move 'struct reggroup' into the reggroups.h header.  Remove the
reggroup_name and reggroup_type accessor functions, and just use the
name/type member functions within 'struct reggroup', update all uses
of these removed functions.

There should be no user visible changes after this commit.

gdb/infcmd.c

@@ -2301,7 +2301,7 @@ registers_info (const char *addr_exp, int fpregs)
 	    /* Don't bother with a length check.  Should the user
 	       enter a short register group name, go with the first
 	       group that matches.  */
-	    if (strncmp (start, reggroup_name (g), end - start) == 0)
+	    if (strncmp (start, g->name (), end - start) == 0)
 	      {
 		group = g;
 		break;