Add host_cpu_load_info on Apple

[stepancheg]

Snippet from host_info.h:

struct host_cpu_load_info {             /* number of ticks while running... */
    natural_t       cpu_ticks[CPU_STATE_MAX]; /* ... in the given mode */
};

typedef struct host_cpu_load_info       host_cpu_load_info_data_t;
typedef struct host_cpu_load_info       *host_cpu_load_info_t;

[rustbot]

r? @tgross35

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