Add translated embedder syscall hook

[doanbaotrung]

Translated x86_64 guests cannot issue the AArch64 HVC instruction used by the existing embedder extension ABI. This prevents those guests from using embedder-provided services such as the graphics bridge.

Add a private elfuse pseudo-syscall as the translated guest counterpart to HVC 6. The syscall keeps the existing hvc6_handler ABI instead of adding a second embedder callback path, so native and translated guests can share the same dispatch logic.

Use syscall number 999 as an internal ABI value between the translated guest shim and elfuse. This is not a Linux syscall number. It is chosen outside the current generic Linux syscall range to avoid colliding with normal guest syscalls, but it is not reserved by Linux and must remain private to elfuse.

If no embedder handler is registered, dispatch falls back to normal syscall handling.

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Summary by cubic

Adds a private elfuse pseudo-syscall so translated x86_64 guests can call the embedder ABI equivalent to AArch64 HVC 6, enabling features like the graphics bridge. Reuses the existing hvc6_handler so native and translated guests share the same dispatch.

• New Features
  • Introduces ELFUSE_NR_EMBEDDER_HVC6 (999) as a private pseudo-syscall; not a Linux syscall.
  • Build sets the number in mk/config.mk and passes -DELFUSE_NR_EMBEDDER_HVC6=$(ELFUSE_NR_EMBEDDER_HVC6), allowing overrides.
  • Syscall hook: if a handler is registered, reads X0 (call id) and X1 (GVA of uint64_t args[8]); on read failure returns -EFAULT; else calls hvc6_handler. If no handler, falls back to normal syscall handling.

^{Written for commit 262954b. Summary will update on new commits.}

[doanbaotrung]

Hi @Max042004
Please help to review this

[jserv]

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