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/runtime/entrypoints/quick/quick_trampoline_entrypoints.cc b/runtime/entrypoints/quick/quick_trampoline_entrypoints.cc
index 345b0ad..838427f 100644
--- a/runtime/entrypoints/quick/quick_trampoline_entrypoints.cc
+++ b/runtime/entrypoints/quick/quick_trampoline_entrypoints.cc
@@ -89,7 +89,7 @@
   // | LR         |
   // | X29        |
   // |  :         |
-  // | X19        |
+  // | X20        |
   // | X7         |
   // | :          |
   // | X1         |