winnt.h: fix ARM64 CONTEXT struct layout mismatch

The fallback CONTEXT struct for ARM64 had multiple structural issues:
- ContextFlags was DWORD64 (8 bytes) instead of ULONG (4 bytes)
- Missing Cpsr field entirely
- Missing DECLSPEC_ALIGN(16) attribute
- X registers as simple array X[29] instead of union with named struct X[31]

These mismatches caused incorrect struct size and field offsets, leading to
register corruption when used with Windows APIs like GetThreadContext or
RtlRestoreContext.

The fallback struct now matches the official ARM64_NT_CONTEXT layout exactly,
ensuring binary compatibility with Windows ARM64 system calls.

win32/include/winapi/winnt.h

@@ -2072,11 +2072,45 @@ typedef DWORD LCID;
 
 #if defined(__aarch64__) && !defined(_ARM64_CONTEXT_DECLARED)
 #define _ARM64_CONTEXT_DECLARED
-    typedef struct _CONTEXT {
-      DWORD64 ContextFlags;
-      DWORD64 X[29];
-      DWORD64 Fp;
-      DWORD64 Lr;
+    typedef struct DECLSPEC_ALIGN(16) _CONTEXT {
+      ULONG ContextFlags;
+      ULONG Cpsr;
+      union {
+        struct {
+          DWORD64 X0;
+          DWORD64 X1;
+          DWORD64 X2;
+          DWORD64 X3;
+          DWORD64 X4;
+          DWORD64 X5;
+          DWORD64 X6;
+          DWORD64 X7;
+          DWORD64 X8;
+          DWORD64 X9;
+          DWORD64 X10;
+          DWORD64 X11;
+          DWORD64 X12;
+          DWORD64 X13;
+          DWORD64 X14;
+          DWORD64 X15;
+          DWORD64 X16;
+          DWORD64 X17;
+          DWORD64 X18;
+          DWORD64 X19;
+          DWORD64 X20;
+          DWORD64 X21;
+          DWORD64 X22;
+          DWORD64 X23;
+          DWORD64 X24;
+          DWORD64 X25;
+          DWORD64 X26;
+          DWORD64 X27;
+          DWORD64 X28;
+          DWORD64 Fp;
+          DWORD64 Lr;
+        } DUMMYSTRUCTNAME;
+        DWORD64 X[31];
+      } DUMMYUNIONNAME;
       DWORD64 Sp;
       DWORD64 Pc;
       ARM64_NT_NEON128 V[32];