harfbuzz: update to 1.8.8.

Overview of changes leading to 1.8.8
Tuesday, August 14, 2018
====================================
- Fix hb-icu crash on architectures where compare_exchange_weak() can
  fail falsely.  This bug was introduced in 1.8.4.
  https://bugs.chromium.org/p/chromium/issues/detail?id=873568
- More internal refactoring of atomic operations and singletons.
- API changes:
  The following functions do NOT reference their return value before
  returning:
  * hb_unicode_funcs_get_default()
  * hb_glib_get_unicode_funcs()
  * hb_icu_get_unicode_funcs()
  This is consistent with their naming ("get", instead of "reference")
  as well as how they are used in the wild (ie. no one calls destroy()
  on their return value.)

(wiz)