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/libunwindstack/tests/RegsTest.cpp b/libunwindstack/tests/RegsTest.cpp
index e6de56a..f549a50 100644
--- a/libunwindstack/tests/RegsTest.cpp
+++ b/libunwindstack/tests/RegsTest.cpp
@@ -58,6 +58,8 @@
       : RegsImpl<TypeParam>(total_regs, regs_sp, return_loc) {}
   virtual ~RegsTestImpl() = default;
 
+  uint32_t MachineType() override { return 0; }
+
   uint64_t GetAdjustedPc(uint64_t, Elf*) override { return 0; }
   void SetFromRaw() override {}
   bool StepIfSignalHandler(uint64_t, Elf*, Memory*) override { return false; }