selftests/bpf: Fix btf_dump test under new clang

New clang version changed ([0]) type name in dwarf from "long int" to "long",
this is causing btf_dump tests to fail.

  [0] https://github.com/llvm/llvm-project/commit/f6a561c4d6754b13165a49990e8365d819f64c86

Signed-off-by: Yucong Sun <sunyucong@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211008173139.1457407-1-fallentree@fb.com