Print the correct system memory in use with +RTS -s (#17158)

Changed to use stats.max_mem_in_use_bytes instead of stats.max_live_bytes in rts/Stats.c to print the system memory in use. This is in line with what the RTS prints with -t option.

Also changed the message to print MiB total memory in use instead of MB total memory in use. We are dividing by 1024, so MiB is the correct usage.

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