Introduce a Marking Register in ARM64 code generation.

When generating code for ARM64, maintain the status of
Thread::Current()->GetIsGcMarking() in register X20,
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: ARM64 device boot test
Bug: 37707231
Change-Id: Ibe9bc5c99a2176b0a0476e9e9ad7fcc9f745017b
diff --git a/compiler/optimizing/intrinsics_arm64.cc b/compiler/optimizing/intrinsics_arm64.cc
index 37d7981..aec1ec7 100644
--- a/compiler/optimizing/intrinsics_arm64.cc
+++ b/compiler/optimizing/intrinsics_arm64.cc
@@ -205,7 +205,7 @@
     // TODO: Load the entrypoint once before the loop, instead of
     // loading it at every iteration.
     int32_t entry_point_offset =
-        CodeGenerator::GetReadBarrierMarkEntryPointsOffset<kArm64PointerSize>(tmp_.reg());
+        Thread::ReadBarrierMarkEntryPointsOffset<kArm64PointerSize>(tmp_.reg());
     // This runtime call does not require a stack map.
     codegen->InvokeRuntimeWithoutRecordingPcInfo(entry_point_offset, instruction_, this);
     codegen->GetAssembler()->MaybePoisonHeapReference(tmp_reg);