gc/default/default.c: don't call `malloc_usable_size` when hint is present

Depending on the allocator, `malloc_usable_size` may be very cheap or quite
expensive. On `macOS` for instance, it's about as expensive as `malloc`.

In many case we call `objspace_malloc_size` with as size we initially
requested as `hint`. The real usable size may be a few bytes bigger,
but since we only use that data to feed GC heuristics, I don't think
it's very important to be perfectly accurate.

It would make sense to call `malloc_usable_size` after growing a String
or Array to use the extra capacity, but here we don't do that, so
the call isn't worth its cost.

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