Add BTI support to amd64 coroutine.

Index: coroutine/amd64/Context.S
--- coroutine/amd64/Context.S.orig
+++ coroutine/amd64/Context.S
@@ -7,11 +7,13 @@
 
 #define TOKEN_PASTE(x,y) x##y
 #define PREFIXED_SYMBOL(prefix,name) TOKEN_PASTE(prefix,name)
+#include <cet.h>
 
 .text
 
 .globl PREFIXED_SYMBOL(SYMBOL_PREFIX,coroutine_transfer)
 PREFIXED_SYMBOL(SYMBOL_PREFIX,coroutine_transfer):
+	_CET_ENDBR
 
 	# Save caller state
 	pushq %rbp
