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/code_generator_x86.cc b/compiler/optimizing/code_generator_x86.cc
index 79fccfe..af0e646 100644
--- a/compiler/optimizing/code_generator_x86.cc
+++ b/compiler/optimizing/code_generator_x86.cc
@@ -509,8 +509,7 @@
     //
     //   rX <- ReadBarrierMarkRegX(rX)
     //
-    int32_t entry_point_offset =
-        CodeGenerator::GetReadBarrierMarkEntryPointsOffset<kX86PointerSize>(ref_reg);
+    int32_t entry_point_offset = Thread::ReadBarrierMarkEntryPointsOffset<kX86PointerSize>(ref_reg);
     // This runtime call does not require a stack map.
     x86_codegen->InvokeRuntimeWithoutRecordingPcInfo(entry_point_offset, instruction_, this);
     __ jmp(GetExitLabel());
@@ -595,8 +594,7 @@
     //
     //   rX <- ReadBarrierMarkRegX(rX)
     //
-    int32_t entry_point_offset =
-        CodeGenerator::GetReadBarrierMarkEntryPointsOffset<kX86PointerSize>(ref_reg);
+    int32_t entry_point_offset = Thread::ReadBarrierMarkEntryPointsOffset<kX86PointerSize>(ref_reg);
     // This runtime call does not require a stack map.
     x86_codegen->InvokeRuntimeWithoutRecordingPcInfo(entry_point_offset, instruction_, this);
 
@@ -7153,7 +7151,7 @@
 
       // Test the entrypoint (`Thread::Current()->pReadBarrierMarkReg ## root.reg()`).
       const int32_t entry_point_offset =
-          CodeGenerator::GetReadBarrierMarkEntryPointsOffset<kX86PointerSize>(root.reg());
+          Thread::ReadBarrierMarkEntryPointsOffset<kX86PointerSize>(root.reg());
       __ fs()->cmpl(Address::Absolute(entry_point_offset), Immediate(0));
       // The entrypoint is null when the GC is not marking.
       __ j(kNotEqual, slow_path->GetEntryLabel());