selftests/exec: test the transparent binfmt_misc mode

Verify the identity a transparent dispatch constructs, from both
activation paths.

- binfmt_misc_transparent: registers a magic entry with the static 'T'
  flag and execs a matched binary with arguments.

- binfmt_misc_bpf: a handler whose load program sets
  BPF_BINPRM_TRANSPARENT.

Both dispatch to a shared asserting interpreter that runs in place of
the binary and checks the contract from the inside:

- AT_FLAGS carries AT_FLAGS_TRANSPARENT_INTERP
- AT_EXECFD refers to the very inode of the binary
- /proc/self/exe resolves to the binary
- argv and /proc/self/cmdline are exactly what the caller passed with
  nothing spliced in
- comm is the binary's basename
- the binary is write-denied while it runs

The static test also validates the registration. 'T' combined with 'P'
must be rejected. A kernel that does not know 'T' turns the test into a
skip. The asserting interpreter and the static test build without the
bpf toolchain so the core transparent semantics stay covered on systems
where the bpf cases are skipped.

The flag support probe, the canonical payload argv with the
run_payload() helper that execs it, and the identity assertions (exe
link, comm, write denial) live in binfmt_misc_common.h; the loader
substitution test reuses all of them.

Link: https://patch.msgid.link/20260721-work-bpf-binfmt_misc-ptinterp-v2-13-e57866e4ae0f@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

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