[NTOS:KD64] kdx64.c: Implement KdpAllowDisable() the same as in x86 (reactos#7538)

AMD64 has the same DR7 register as x86 with the same bits meanings,
thus the same implementation can be used.

References:

- https://en.wikipedia.org/wiki/X86_debug_register#DR7_-_Debug_control

- AMD64 Architecture Programmer’s Manual, Volume 2: System Programming
  https://www.amd.com/content/dam/amd/en/documents/processor-tech-docs/programmer-references/24593.pdf
  Section "13.1.1.4 Debug-Control Register (DR7)"
  pgs. 393-396 (pgs. 455-458 of the PDF)

- Intel® 64 and IA-32 Architectures Software Developer’s Manual,
  Volume 3 (3A, 3B, 3C, & 3D): System Programming Guide
  https://www.intel.com/content/www/us/en/developer/articles/technical/intel-sdm.html
  Section "19.2.4 Debug Control Register (DR7)" (pgs. 644-646)
  Section "19.2.6 Debug Registers and Intel® 64 Processors" (pg. 647)

Author: HBelusca

ntoskrnl/kd64/amd64/kdx64.c

@@ -363,8 +363,21 @@ NTSTATUS
 NTAPI
 KdpAllowDisable(VOID)
 {
-    UNIMPLEMENTED;
-    return STATUS_ACCESS_DENIED;
+    ULONG i;
+
+    /* Loop every processor */
+    for (i = 0; i < KeNumberProcessors; i++)
+    {
+        PKPROCESSOR_STATE ProcessorState = &KiProcessorBlock[i]->ProcessorState;
+
+        /* If any processor breakpoints are active,
+         * we can't allow running without a debugger */
+        if (ProcessorState->SpecialRegisters.KernelDr7 & 0xFF)
+            return STATUS_ACCESS_DENIED;
+    }
+
+    /* No processor breakpoints, allow disabling the debugger */
+    return STATUS_SUCCESS;
 }
 
 /* EOF */