exec: add AT_FLAGS_TRANSPARENT_INTERP

A transparent binfmt_misc dispatch hands the binary to the interpreter
through AT_EXECFD and leaves the argument vector exactly as the caller
built it. The loader on the receiving end has to know which contract it
got.

On the classic 'O'/'C' entries the binary's path is spliced into the
argument vector and the loader consumes arguments. In transparent mode
nothing was spliced and argv belongs entirely to the program. This
cannot be inferred from AT_EXECFD alone. Raise a new AT_FLAGS bit
following the AT_FLAGS_PRESERVE_ARGV0 precedent added for qemu-user in
commit 2347961b11d4 ("binfmt_misc: pass binfmt_misc flags to the
interpreter").

The bit also announces that mm->exe_file names the binary rather than
the interpreter (added in the next commit). A loader that sees the bit
may finish the identity polish by fixing up AT_PHDR/AT_ENTRY/AT_BASE in
saved_auxv and fix the code/data markers via one uncapped PR_SET_MM_MAP
once it has mapped the binary. I've got glibc patches for this as well
but it's useful for any loader.

BINPRM_FLAGS_TRANSPARENT_INTERP carries the mode from binfmt_misc to the
ELF loaders. Both had their own copy of the AT_FLAGS translation, so
give them one bprm_at_flags() to share instead of a second copy that can
drift. Nothing sets the bprm flag yet.

Link: https://patch.msgid.link/20260721-work-bpf-binfmt_misc-ptinterp-v2-8-e57866e4ae0f@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

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