binfmt_misc: let a bpf handler request loader substitution

Give bpf handlers the per-exec equivalent of the static 'L' flag. A
load program that sets BPF_BINPRM_LOADER has its selected interpreter
substituted for the binary's PT_INTERP instead of run with the binary
as payload. The binary otherwise executes as a fully native exec.

A single handler can now grade its dispatch per binary: native-arch ELF
with PT_INTERP gets loader substitution for full native identity.
Anything else, such as foreign arch, static, non-ELF can use transparent
or classic dispatch. The load program can read the binary's ELF header
from bprm->buf to make that call.

Link: https://patch.msgid.link/20260721-work-bpf-binfmt_misc-ptinterp-v2-19-e57866e4ae0f@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

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