core/arm: Remove usage of global GetCurrentThread()

Now both CPU backends go through their referenced system instance to
obtain the current thread.

src/core/arm/dynarmic/arm_dynarmic.cpp

@@ -15,6 +15,7 @@
 #include "core/core_timing_util.h"
 #include "core/gdbstub/gdbstub.h"
 #include "core/hle/kernel/process.h"
+#include "core/hle/kernel/scheduler.h"
 #include "core/hle/kernel/svc.h"
 #include "core/hle/kernel/vm_manager.h"
 #include "core/memory.h"
@@ -87,7 +88,7 @@ public:
             if (GDBStub::IsServerEnabled()) {
                 parent.jit->HaltExecution();
                 parent.SetPC(pc);
-                Kernel::Thread* thread = Kernel::GetCurrentThread();
+                Kernel::Thread* const thread = parent.system.CurrentScheduler().GetCurrentThread();
                 parent.SaveContext(thread->GetContext());
                 GDBStub::Break();
                 GDBStub::SendTrap(thread, 5);

src/core/arm/unicorn/arm_unicorn.cpp

@@ -9,6 +9,7 @@
 #include "core/arm/unicorn/arm_unicorn.h"
 #include "core/core.h"
 #include "core/core_timing.h"
+#include "core/hle/kernel/scheduler.h"
 #include "core/hle/kernel/svc.h"
 
 namespace Core {
@@ -177,7 +178,7 @@ void ARM_Unicorn::ExecuteInstructions(std::size_t num_instructions) {
             uc_reg_write(uc, UC_ARM64_REG_PC, &last_bkpt.address);
         }
 
-        Kernel::Thread* thread = Kernel::GetCurrentThread();
+        Kernel::Thread* const thread = system.CurrentScheduler().GetCurrentThread();
         SaveContext(thread->GetContext());
         if (last_bkpt_hit || GDBStub::IsMemoryBreak() || GDBStub::GetCpuStepFlag()) {
             last_bkpt_hit = false;