Add tpebs retire latency mean as default value in event JSON files

[weilinwa]

This change add support for default retire latency values in converted perf JSON files. It loads platform specific retire latency default value in JSON file and insert the mean retire latency data into event JSON files as a new field to each event.

An example as following (the xxxx will be float value in real files):

    {
        "BriefDescription": "Retired load instructions with locked access.",
        "Counter": "0,1,2,3",
        "Data_LA": "1",
        "EventCode": "0xd0",
        "EventName": "MEM_INST_RETIRED.LOCK_LOADS",
        "PEBS": "1",
        "PublicDescription": "Counts retired load instructions with locked access.",
        "RetireLatencyMean": xxxx,
        "SampleAfterValue": "100007",
        "UMask": "0x21",
        "Unit": "cpu_core"
    }