target: get_gdb_arch() accepts target via const pointer

The function in question does not need to change target state. It is a
target-type-dependant function, however, IMHO, it is safe to assume that
any target type would not need to change type-independant state of a
target to figure out the arch.

Change-Id: I607cb3aee6529cd5a97bc1200a0226cf6ef43caf
Signed-off-by: Evgeniy Naydanov <evgeniy.naydanov@syntacore.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8093
Tested-by: jenkins
Reviewed-by: Jan Matyas <jan.matyas@codasip.com>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>

src/target/xtensa/xtensa.c

@@ -3442,7 +3442,7 @@ void xtensa_target_deinit(struct target *target)
 	free(xtensa->core_config);
 }
 
-const char *xtensa_get_gdb_arch(struct target *target)
+const char *xtensa_get_gdb_arch(const struct target *target)
 {
 	return "xtensa";
 }