ntdll iOS: fill missing control registers in a cross-thread GetThread…

[bahacan16]

…Context

Problem

Ghost of Tsushima froze on a loading screen. The log showed RtlpWaitForCriticalSection ... blocked by <tid>: the blocking thread was calling GetThreadContext on another thread thousands of times, every answer rip=0 rsp=0 flags=00100002 (no CONTEXT_CONTROL), while it held a critical section the main thread was waiting for.

Cause

For a cross-thread NtGetContextThread the server sometimes has no native capture of the target (have_native = 0) and answers with the integer registers only. The ARM64EC wrapper then hands the x64 caller a context with Rip = Rsp = 0 and no CONTEXT_CONTROL, and a caller that samples another thread's instruction pointer (profilers, job systems, watchdogs) keeps retrying.

Change

build/ntdll-unix/signal_arm64_ios.c: when the server reply lacks the control registers (or has Pc = 0) and the caller asked for them, get_context_from_amd64_area fills them -- and the integer registers, if requested -- from the target's last saved x64 state, TEB->ChpeV2CpuAreaInfo->ContextAmd64 (written whenever the thread leaves emulated code). The values are returned in the ARM64EC register mapping, so the wrapper's context_arm_to_x64() turns them back into x64 registers; X23/X28 are cleared so the wrapper does not substitute an emulator frame's Pc/Sp. Only threads of the calling process are handled (the TEB pointer is read in this address space); nothing changes when the server's answer is complete. The first 8 uses log [ctx] get handle=....

Evidence

Ghost of Tsushima, iPhone 17 Pro Max, iOS 27: the frozen loading screen's log had the blocked critical section and the stream of rip=0 rsp=0 contexts described above; the change shipped in the fork after that log.

Notes / risks

• Not confirmed on device. No later log recorded the freeze again, but none recorded the [ctx] line either, so whether this was the fix is unproven. Treat it as a correctness improvement: a stale but real Rip/Rsp instead of zeros.
• The saved state is the thread's last exit from emulated code, not a suspended snapshot; a running thread's registers may have moved on (Windows callers are expected to suspend first).
• If ContextAmd64 is empty or Pc/Sp are 0, the server's answer is returned unchanged (still without CONTEXT_CONTROL).
• Split out of the D3D12 tessellation topic; unrelated to it.

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