Add implicit checks for x86_64 architecture.

This combines the x86 and x86_64 fault handlers into one.  It also
merges in the change to the entrypoints for X86_64.

Replaces generic instruction length calculator with one that only
works with the specific instructions we use.

Bug: 16256184

Change-Id: I1e8ab5ad43f46060de9597615b423c89a836035c
Signed-off-by: Chao-ying Fu <chao-ying.fu@intel.com>
diff --git a/runtime/runtime.cc b/runtime/runtime.cc
index fe877d5..e0c0d63 100644
--- a/runtime/runtime.cc
+++ b/runtime/runtime.cc
@@ -621,6 +621,7 @@
     case kThumb2:
     case kX86:
     case kArm64:
+    case kX86_64:
       implicit_null_checks_ = true;
       implicit_so_checks_ = true;
       break;