ARM64: Update code after the VIXL 1.6 release.

We now leave the assembler buffer management to VIXL.

Change-Id: Ieefe83cf5cf5e1ab8c924b0e7dc03af6a55053ae
diff --git a/compiler/utils/arm64/constants_arm64.h b/compiler/utils/arm64/constants_arm64.h
index 0cbbb1e..ffb54d3 100644
--- a/compiler/utils/arm64/constants_arm64.h
+++ b/compiler/utils/arm64/constants_arm64.h
@@ -31,8 +31,7 @@
 
 constexpr unsigned int kJniRefSpillRegsSize = 11;
 
-// Vixl buffer size.
-constexpr size_t kBufferSizeArm64 = 4096*2;
+constexpr size_t kArm64BaseBufferSize = 4096;
 
 }  // namespace arm64
 }  // namespace art