ARM64: Move xSELF from x18 to x19.

This patch moves xSELF to callee saved x19 and removes support for
ETR (external thread register), previously used across native calls.

Change-Id: Icee07fbb9292425947f7de33d10a0ddf98c7899b
Signed-off-by: Serban Constantinescu <serban.constantinescu@linaro.org>
diff --git a/disassembler/disassembler_arm64.cc b/disassembler/disassembler_arm64.cc
index d195efc..348b2a5 100644
--- a/disassembler/disassembler_arm64.cc
+++ b/disassembler/disassembler_arm64.cc
@@ -31,8 +31,7 @@
 // runtime/arch/arm64/registers_arm64.h. We do not include that file to
 // avoid a dependency on libart.
 enum {
-  TR  = 18,
-  ETR = 21,
+  TR  = 19,
   IP0 = 16,
   IP1 = 17,
   FP  = 29,