x86: amend cpu_has_xen_{ibt,shstk}

... to evaluate to false at compile-time when the respective Kconfig
control is off, thus allowing the compiler to eliminate then-dead code.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/arch/x86/include/asm/cpufeature.h

@@ -215,8 +215,10 @@ static inline bool boot_cpu_has(unsigned int feat)
 #define cpu_has_lfence_dispatch boot_cpu_has(X86_FEATURE_LFENCE_DISPATCH)
 #define cpu_has_nscb            boot_cpu_has(X86_FEATURE_NSCB)
 #define cpu_has_xen_lbr         boot_cpu_has(X86_FEATURE_XEN_LBR)
-#define cpu_has_xen_shstk       boot_cpu_has(X86_FEATURE_XEN_SHSTK)
-#define cpu_has_xen_ibt         boot_cpu_has(X86_FEATURE_XEN_IBT)
+#define cpu_has_xen_shstk       (IS_ENABLED(CONFIG_XEN_SHSTK) && \
+                                 boot_cpu_has(X86_FEATURE_XEN_SHSTK))
+#define cpu_has_xen_ibt         (IS_ENABLED(CONFIG_XEN_IBT) && \
+                                 boot_cpu_has(X86_FEATURE_XEN_IBT))
 
 #define cpu_has_msr_tsc_aux     (cpu_has_rdtscp || cpu_has_rdpid)