fix compilation check for `malloc_usable_size()` presence

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### Expected behavior

When non-default allocator is chosen (jemalloc or tcmalloc), the presence of `malloc_usable_size()` in that particular allocator should be used to decide the value of `-DROCKSDB_MALLOC_USABLE_SIZE`.

### Actual behavior

Regardless of the configured allocator, the compilation checks for the presence of `malloc_usable_size()` in the default allocator: https://github.com/facebook/rocksdb/blob/0a0151fb99b306dc0b1f6b22df8073f8084da795/build_tools/build_detect_platform#L412-L425. This causes runtime crashes when the default allocator supports `malloc_usable_size()` but the runtime allocator does not (e.g., older versions of tcmalloc).