Address GCC8 warning

The bootstrapping allocator needs to perform a memcpy to bypass the
removed move constructors on std::atomic.  This is safe as there is no
concurrency at this point, but GCC is unhappy with this.

This commit moves CI to GCC8 and disables this warning for that line.

src/ds/defines.h

@@ -18,6 +18,12 @@
 #  define SNMALLOC_PURE __attribute__((const))
 #endif
 
+#if !defined(__clang__) && defined(__GNUC__)
+#  if __GNUC__ >= 8
+#    define GCC_VERSION_EIGHT_PLUS
+#  endif
+#endif
+
 #ifndef __has_builtin
 #  define __has_builtin(x) 0
 #endif

src/mem/largealloc.h

@@ -102,9 +102,18 @@ namespace snmalloc
       MemoryProviderStateMixin<PAL>* allocated =
         local.alloc_chunk<MemoryProviderStateMixin<PAL>, 1>();
 
+#ifdef GCC_VERSION_EIGHT_PLUS
+#  pragma GCC diagnostic push
+#  pragma GCC diagnostic ignored "-Wclass-memaccess"
+#endif
       // Put temporary allocator we have used, into the permanent storage.
-      // memcpy is safe as this is entirely single threaded.
+      // memcpy is safe as this is entirely single threaded: the move
+      // constructors were removed as unsafe to move std::atomic in a
+      // concurrent setting.
       memcpy(allocated, &local, sizeof(MemoryProviderStateMixin<PAL>));
+#ifdef GCC_VERSION_EIGHT_PLUS
+#  pragma GCC diagnostic pop
+#endif
 
       return allocated;
     }