Introduce a Marking Register in ARM code generation.

When generating code for ARM, maintain the status of
Thread::Current()->GetIsGcMarking() in register R8,
dubbed MR (Marking Register), and check the value of that
register (instead of loading and checking a read barrier
marking entrypoint) in read barriers.

Test: m test-art-target
Test: m test-art-target with tree built with ART_USE_READ_BARRIER=false
Test: m test-art-host-gtest
Test: ARM device boot test
Bug: 37707231
Change-Id: I30b44254460d0bbb9f1b2adc65eca52ca3de3f53
diff --git a/compiler/utils/assembler_thumb_test.cc b/compiler/utils/assembler_thumb_test.cc
index 4e9b619..759ed38 100644
--- a/compiler/utils/assembler_thumb_test.cc
+++ b/compiler/utils/assembler_thumb_test.cc
@@ -1643,6 +1643,10 @@
 #define __ assembler.
 
 TEST_F(ArmVIXLAssemblerTest, VixlJniHelpers) {
+  // Run the test only with Baker read barriers, as the expected
+  // generated code contains a Marking Register refresh instruction.
+  TEST_DISABLED_WITHOUT_BAKER_READ_BARRIERS();
+
   const bool is_static = true;
   const bool is_synchronized = false;
   const bool is_critical_native = false;