bpf: Prepare verifier logs for upcoming kfunc stack arguments

This change prepares verifier log reporting for upcoming kfunc stack
argument support.

Currently verifier log code mostly assumes that an argument can be
described directly by a register number. That works for arguments
passed in `R1` to `R5`, but it does not work once kfunc arguments
can also be passed on the stack.

Introduce an opaque `argno_t` type that encodes both register-based
and arg-based references. Four helpers form the interface:

  - argno_from_reg(regno): create from a register number
  - argno_from_arg(arg):   create from a 1-based arg number
  - reg_from_argno(a):     extract register number, or -1
  - arg_from_argno(a):     extract arg number, or -1

reg_arg_name() converts an argno_t to a human-readable string for
verifier logs: "R%d" for register arguments, or "*(R11-off)" for
stack arguments beyond R5.

Update selftests accordingly.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20260423033501.2539667-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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