gdb: add type::endianity_is_not_default / type::set_endianity_is_not_default

Add the `endianity_is_not_default` and `set_endianity_is_not_default`
methods on `struct type`, in order to remove the
`TYPE_ENDIANITY_NOT_DEFAULT` 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) <endianity_is_not_default,
	set_endianity_is_not_default>: New methods.
	(TYPE_ENDIANITY_NOT_DEFAULT): Use
	type::endianity_is_not_default, change all write call sites to
	use type::set_endianity_is_not_default.

Change-Id: I67acd68fcdae424d7e4a601afda78612ad5d92db