Fix mac build.

bug:29321958
Change-Id: Ibd51d408dd97876eb3237df5606dbe9290ca7254
diff --git a/runtime/arch/x86/fault_handler_x86.cc b/runtime/arch/x86/fault_handler_x86.cc
index 667d200..24e3a0d 100644
--- a/runtime/arch/x86/fault_handler_x86.cc
+++ b/runtime/arch/x86/fault_handler_x86.cc
@@ -36,6 +36,7 @@
 #define CTX_EIP uc_mcontext->__ss.__rip
 #define CTX_EAX uc_mcontext->__ss.__rax
 #define CTX_METHOD uc_mcontext->__ss.__rdi
+#define CTX_RDI uc_mcontext->__ss.__rdi
 #define CTX_JMP_BUF uc_mcontext->__ss.__rdi
 #else
 // 32 bit mac build.