gdb: add type::has_no_signedness / type::set_has_no_signedness

gdb: add type::has_no_signedness / type::set_has_no_signedness

Add the `has_no_signedness` and `set_has_no_signednes` methods on `struct
type`, in order to remove the `TYPE_NOSIGN` macro.  In this patch, the macro is
changed to use the getter, so all the call sites of the macro that are used as
a setter are changed to use the setter method directly.  The next patch will
remove the macro completely.

gdb/ChangeLog:

	* gdbtypes.h (struct type) <has_no_signedness,
	set_has_no_signedness>: New methods.
	(TYPE_NOSIGN): Use type::has_no_signedness, change all write
	call sites to use type::set_has_no_signedness.

Change-Id: I80d8e774316d146fbd814b2928ad5392bada39d5