Vendor deprecated btf_ext APIs and structs

Vendor deprecated btf_ext APIs and structs

Some btf_ext-related APIs in libbpf are being deprecated because they
make incorrect assumptions. They're being used only by bcc currently, so
vendor them before they get deleted.

After / as part of #3660, may need to revisit the incorrect assumptions
being made here.

The functions and structs were ripped directly from libbpf with minimal
changes:
  * Change void* arithmetic to uint8_t
  * __u32 -> uint32_t and similar
  * Add a wrapping namespace
  * `rec_size` functions were not needed - just grab the rec_size
directly since type is no longer opaque to bcc