libunwindstack: make machine type a property of Regs.

There are no actual users of the machine type output parameter to
Regs::RemoteGet. The concrete implementations of Regs know what machine
type they represent anyway, so provide an accessor to query.

Test: treehugger
Test: libunwindstack tests on 32/64-bit host, hikey960
Change-Id: Ia25910531d36c41b2b6919f154cfa914aae63117
diff --git a/libbacktrace/UnwindStack.cpp b/libbacktrace/UnwindStack.cpp
index e79bca3..3b2f38e 100644
--- a/libbacktrace/UnwindStack.cpp
+++ b/libbacktrace/UnwindStack.cpp
@@ -160,7 +160,8 @@
     // one extra function call appearing in the unwind.
     unwindstack::RegsGetLocal(regs.get());
   } else {
-    regs.reset(unwindstack::Regs::CreateFromUcontext(unwindstack::Regs::GetMachineType(), ucontext));
+    regs.reset(
+        unwindstack::Regs::CreateFromUcontext(unwindstack::Regs::CurrentMachineType(), ucontext));
   }
 
   error_ = BACKTRACE_UNWIND_NO_ERROR;
@@ -177,10 +178,10 @@
 bool UnwindStackPtrace::Unwind(size_t num_ignore_frames, ucontext_t* context) {
   std::unique_ptr<unwindstack::Regs> regs;
   if (context == nullptr) {
-    uint32_t machine_type;
-    regs.reset(unwindstack::Regs::RemoteGet(Tid(), &machine_type));
+    regs.reset(unwindstack::Regs::RemoteGet(Tid()));
   } else {
-    regs.reset(unwindstack::Regs::CreateFromUcontext(unwindstack::Regs::GetMachineType(), context));
+    regs.reset(
+        unwindstack::Regs::CreateFromUcontext(unwindstack::Regs::CurrentMachineType(), context));
   }
 
   error_ = BACKTRACE_UNWIND_NO_ERROR;