x86/Kconfig: Add CONFIG_CALL_THUNKS

JIRA: https://issues.redhat.com/browse/RHEL-255

commit 8f7c0d8
Author: Thomas Gleixner <tglx@linutronix.de>
Date:   Thu Sep 15 13:11:17 2022 +0200

    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

Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>

Author: joe-lawrence

arch/x86/Kconfig

@@ -2476,6 +2476,14 @@ 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