Add BTI support to arm64 coroutine.

Index: coroutine/arm64/Context.S
--- coroutine/arm64/Context.S.orig
+++ coroutine/arm64/Context.S
@@ -20,6 +20,7 @@
 
 .global PREFIXED_SYMBOL(SYMBOL_PREFIX,coroutine_transfer)
 PREFIXED_SYMBOL(SYMBOL_PREFIX,coroutine_transfer):
+	bti c
 
 	# Make space on the stack for caller registers
 	sub sp, sp, 0xb0
