internal/dxcore: Fix build on 32-bit targets

[rautyrauty]

Description

dxcore.getAdapter() reinterprets the C adapter list through a
(*[1 << 30]C.struct_dxcore_adapter) array pointer. On ILP32 targets the
struct is 76 bytes, so that array type is larger than the whole address
space and the compiler rejects the type itself:

internal/dxcore/dxcore.go:55:2: type [1073741824]_Ctype_struct_dxcore_adapter larger than address space
internal/dxcore/dxcore.go:55:2: type [1073741824]_Ctype_struct_dxcore_adapter too large

This breaks every consumer that imports pkg/nvcdi on GOARCH=386 or
GOARCH=arm. We hit it packaging LXD 6.9, which vendors
nvidia-container-toolkit v1.19.1, for the i586 architecture of ALT Linux.

The fix indexes the C array through unsafe.Slice() bounded by
adapterCount, which is the modern replacement for the huge-array idiom
and works on every pointer size. No behaviour change on 64-bit targets.

Checklist

• No secrets, sensitive information, or unrelated changes
• Unit tests passing (make test)
• Lint checks passing (make lint)
• Test cases are added for new code paths — no new code path; this is a
  compile-time fix with no runtime behaviour change
• Commits are signed-off and cryptographically signed

Testing

• GOARCH=386 CGO_ENABLED=1 go build ./internal/dxcore/ fails on main
  with the diagnostic above and passes with this change.
• go build ./internal/dxcore/ ./pkg/nvcdi/ and make test pass on amd64.
• LXD 6.9 with this change applied to its vendored copy builds and passes
  its packaging checks in a clean i586 chroot on ALT Linux Sisyphus.

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[henry118]

@rautyrauty thanks for the patch. LGTM.

Can you please sign your commit?