feat(binary-layout): admit ELF64 AArch64 and name the refused host (#8711)

* feat(binary-layout): admit ELF64 AArch64 and name the refused host

inspect_binary_layout refused every AArch64 ELF, and on a non-x64 host the
refusal read "supports Linux x64 only", which looked like a target refusal
(#8709 ran on arm64 WSL and suspected an e_machine check).

The pwntools adapter now admits little-endian ELF64 AArch64 EXEC/DYN/REL
objects alongside the existing profiles, pwntools must report aarch64/64/
little, and the Evidence subject names arm64. The host refusal now names the
detected platform/arch and says the target was not read; the host check
itself is unchanged.

verify:binary:layout cross-compiles an AArch64 executable and relocatable
object (never executed) and checks sections, symbols and relocations
against GNU readelf, decoding ELF64 r_info as a 32-bit type under a 32-bit
symbol index. The real-binary-layout workflow installs the toolchain.

Refs #8709

* test(binary-layout): refuse RISC-V as the unsupported machine and assert the host refusal path

verify:binary:layout used e_machine 183 (AArch64) as its unsupported
target, which this branch now admits; use EM_RISCV (243) instead.

The unsupported-host provider test now asserts the refusal names this
host's platform/arch and says the target was not read, so reverting the
decoder to the old message fails it.

docs/binary-diagnostics.md

@@ -9,9 +9,11 @@ ordinary binary inspection use the same modular capability.
 ## Bring your own engine
 
 Real verification covers **Linux x64** hosts and **EXEC/DYN/REL** objects in
-these profiles: ELF64 x86-64, ELF32 i386, ELF32 little-endian ARM and ELF32
-little- or big-endian MIPS. Other machines, classes and byte orders, including
-AArch64 and big-endian ARM, return `unsupported_target`. Provide an absolute
+these profiles: ELF64 x86-64, ELF64 little-endian AArch64, ELF32 i386, ELF32
+little-endian ARM and ELF32 little- or big-endian MIPS. Other machines, classes
+and byte orders, including big-endian ARM, return `unsupported_target`. Other
+hosts, including Linux arm64, return `capability_unavailable` naming the
+detected host before any target is read. Provide an absolute
 Python executable whose environment already contains unchanged pwntools 4.15.0,
 pyelftools 0.33 and Unicorn 2.1.2: