add btf support for maps

[yonghong-song]

Added bpf support for maps so map key/value types can be
retrieved by user space introspection tool to pretty
print map key/values.

To associate maps with its key/value types, the below macro

  BPF_ANNOTATE_KV_PAIR(name, type_key, type_val)

is used, similar to usage in kernel tools/testing/selftests/bpf
and tools/lib/bpf. Currently, some map types (e.g, PERF_EVENT_ARRAY)
do not support pretty print. But common [per_cpu] hash/array maps
are supported.

Currently, bcc create maps before llvm compilation.
To support pretty printing of maps, map creation
needs have key/value type id's which can only be
obtained after compilation. Therefore, during rewriting,
fake map fd is used. After compilation, btf is first
loaded and maps are created.

With latest bpf-next and latest trunk llvm,
bpftool is able to pretty print bcc tool map key/values
as below:

  ; running tcptop.py in one window
  $ tcptop.py
  ; running bpftool in another window
  $ bpftool m s
  ...
  343: hash  name ipv4_send_bytes  flags 0x0
          key 16B  value 8B  max_entries 10240  memlock 1003520B
  344: hash  name ipv4_recv_bytes  flags 0x0
          key 16B  value 8B  max_entries 10240  memlock 1003520B
  345: hash  name ipv6_send_bytes  flags 0x0
          key 64B  value 8B  max_entries 10240  memlock 1495040B
  346: hash  name ipv6_recv_bytes  flags 0x0
          key 64B  value 8B  max_entries 10240  memlock 1495040B
  $ bpftool m d id 345
  ...
    },{
        "key": {
            "pid": 5511,
            "saddr": 0x100007fffff00000000000000000000,
            "daddr": 0x100007fffff00000000000000000000,
            "lport": 2378,
            "dport": 52602
        },
        "value": 49
    },{
        "key": {
            "pid": 2823,
            "saddr": 0x4e000000cefa7bb0300000db0124,
            "daddr": 0x60000000cefa7bb0300000db0124,
            "lport": 2406,
            "dport": 49348
        },
        "value": 36
    }
   ...

Signed-off-by: Yonghong Song yhs@fb.com

[yonghong-song]

The tests involving "b" frontends failed. Will add proper support in this round.