bpf, selftest: add test cases for BTF Var and DataSec

Extend test_btf with various positive and negative tests around
BTF verification of kind Var and DataSec. All passing as well:

  # ./test_btf
  [...]
  BTF raw test[4] (global data test #1): OK
  BTF raw test[5] (global data test #2): OK
  BTF raw test[6] (global data test #3): OK
  BTF raw test[7] (global data test #4, unsupported linkage): OK
  BTF raw test[8] (global data test #5, invalid var type): OK
  BTF raw test[9] (global data test #6, invalid var type (fwd type)): OK
  BTF raw test[10] (global data test #7, invalid var type (fwd type)): OK
  BTF raw test[11] (global data test #8, invalid var size): OK
  BTF raw test[12] (global data test #9, invalid var size): OK
  BTF raw test[13] (global data test #10, invalid var size): OK
  BTF raw test[14] (global data test #11, multiple section members): OK
  BTF raw test[15] (global data test #12, invalid offset): OK
  BTF raw test[16] (global data test #13, invalid offset): OK
  BTF raw test[17] (global data test #14, invalid offset): OK
  BTF raw test[18] (global data test #15, not var kind): OK
  BTF raw test[19] (global data test #16, invalid var referencing sec): OK
  BTF raw test[20] (global data test #17, invalid var referencing var): OK
  BTF raw test[21] (global data test #18, invalid var loop): OK
  BTF raw test[22] (global data test #19, invalid var referencing var): OK
  BTF raw test[23] (global data test #20, invalid ptr referencing var): OK
  BTF raw test[24] (global data test #21, var included in struct): OK
  BTF raw test[25] (global data test #22, array of var): OK
  [...]
  PASS:167 SKIP:0 FAIL:0

Signed-off-by: Daniel Borkmann <[email protected]>
Acked-by: Martin KaFai Lau <[email protected]>
Signed-off-by: Alexei Starovoitov <[email protected]>