This fixes build error due to updaded llvm.
bpftrace 0.24.1 had a couple of regressions, so backport fix and
temporarily raise ulimit in test.
A better solution for ulimit will probably make it in 0.24.2 but
that is likely harmless enough in practice so go ahead as is.
Notes:
- add new xxd build dep to fix build
- INSTALL_TOOL_DOCS was removed, .txt files are now comments in scripts
themselves
- USE_SYSTEM_LIBBPF is optional right now but needed on master, just set
it as there is no harm and next auto-update will be less work
27bb09b95e