Change GC_GRANULE_PTRS back to two pointers for CHERI

(fix of commit e1fb227)

Issue #627 (bdwgc).

Some code of bdwgc like `GC_generic_malloc_aligned` expects
value of `GC_GRANULE_PTRS` to be not smaller than 2.

* include/gc/gc_tiny_fl.h [!GC_GRANULE_BYTES
&& __CHERI_PURE_CAPABILITY__] (GC_GRANULE_PTRS): Define to 2.

include/gc/gc_tiny_fl.h

@@ -48,11 +48,7 @@
 #ifdef GC_GRANULE_BYTES
 #  define GC_GRANULE_PTRS (GC_GRANULE_BYTES / GC_SIZEOF_PTR)
 #else
-#  if defined(__CHERI_PURE_CAPABILITY__)
-#    define GC_GRANULE_PTRS 1
-#  else
-#    define GC_GRANULE_PTRS 2 /* in pointers */
-#  endif
+#  define GC_GRANULE_PTRS 2 /* in pointers */
 #  define GC_GRANULE_BYTES (GC_GRANULE_PTRS * GC_SIZEOF_PTR)
 #endif /* !GC_GRANULE_BYTES */