Fix crash at startup related to GC of font entities

Fix crash at startup related to GC of font entities

* src/font.h (GC_FONT_SPEC_P, GC_FONT_ENTITY_P)
(GC_FONT_OBJECT_P, GC_XFONT_SPEC, GC_XFONT_ENTITY)
(GC_XFONT_OBJECT): New macros, for use in garbage collector.
* src/alloc.c (compact_font_cache_entry, compact_font_caches):
Don't ifdef away font cache compaction on NT_GUI, as the problems
which led to that seem to have been solved.
(compact_font_cache_entry): Use GC_FONT_SPEC_P, GC_XFONT_SPEC,
GC_XFONT_ENTITY, and GC_XFONT_OBJECT, instead of their non-GC_
cousins.  (Bug#21999)