x86/Kconfig: Add CONFIG_CALL_THUNKS

In preparation for mitigating the Intel SKL RSB underflow issue in
software, add a new configuration symbol which allows to build the
required call thunk infrastructure conditionally.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20220915111146.849523555@infradead.org

arch/x86/Kconfig

@@ -2446,6 +2446,14 @@ config CC_HAS_SLS
 config CC_HAS_RETURN_THUNK
 	def_bool $(cc-option,-mfunction-return=thunk-extern)
 
+config HAVE_CALL_THUNKS
+	def_bool y
+	depends on RETHUNK && OBJTOOL
+
+config CALL_THUNKS
+	def_bool n
+	select FUNCTION_ALIGNMENT_16B
+
 menuconfig SPECULATION_MITIGATIONS
 	bool "Mitigations for speculative execution vulnerabilities"
 	default y