selftest/bpf: Add tailcall verifier selftest for powerpc64

Verifier testcase result for tailcalls:

# ./test_progs -t verifier_tailcall
#618/1   verifier_tailcall/invalid map type for tail call:OK
#618/2   verifier_tailcall/invalid map type for tail call @unpriv:OK
#618     verifier_tailcall:OK
#619/1   verifier_tailcall_jit/main:OK
#619     verifier_tailcall_jit:OK
Summary: 2/3 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Abhishek Dubey <redacted>

The test output lines in the commit message could benefit from indentation
to more clearly differentiate them as command output. Consider adding a space
or two before each of the test result lines for improved readability.


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See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/27620315552