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.

.github/workflows/real-binary-layout.yml

@@ -55,12 +55,12 @@ jobs:
       - uses: ./.github/actions/portable-build-cache
         with:
           build_cache_artifact: ${{ inputs.build_cache_artifact }}
-      - name: Install ELF32 cross toolchains for source-owned fixtures
+      - name: Install AArch64 and ELF32 cross toolchains for source-owned fixtures
         run: |
           sudo apt-get update
           sudo apt-get install --yes --no-install-recommends \
-            gcc-i686-linux-gnu gcc-arm-linux-gnueabi gcc-mips-linux-gnu gcc-mipsel-linux-gnu \
-            libc6-dev-i386-cross libc6-dev-armel-cross libc6-dev-mips-cross libc6-dev-mipsel-cross
+            gcc-aarch64-linux-gnu gcc-i686-linux-gnu gcc-arm-linux-gnueabi gcc-mips-linux-gnu gcc-mipsel-linux-gnu \
+            libc6-dev-arm64-cross libc6-dev-i386-cross libc6-dev-armel-cross libc6-dev-mips-cross libc6-dev-mipsel-cross
       - name: Acquire exact upstream libraries in an isolated runner environment
         shell: bash
         run: |