x86/cpu: Provide a config option to disable static_cpu_has

This brings .text savings of about ~1.6K when building a tinyconfig. It
is off by default so nothing changes for the default.

Kconfig help text from Josh.
Signed-off-by: NBorislav Petkov <bp@suse.de>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>
Link: http://lkml.kernel.org/r/1449481182-27541-5-git-send-email-bp@alien8.deSigned-off-by: NThomas Gleixner <tglx@linutronix.de>

arch/x86/Kconfig

@@ -349,6 +349,17 @@ config X86_FEATURE_NAMES
 
 	  If in doubt, say Y.
 
+config X86_FAST_FEATURE_TESTS
+	bool "Fast CPU feature tests" if EMBEDDED
+	default y
+	---help---
+	  Some fast-paths in the kernel depend on the capabilities of the CPU.
+	  Say Y here for the kernel to patch in the appropriate code at runtime
+	  based on the capabilities of the CPU. The infrastructure for patching
+	  code at runtime takes up some additional space; space-constrained
+	  embedded systems may wish to say N here to produce smaller, slightly
+	  slower code.
+
 config X86_X2APIC
 	bool "Support x2apic"
 	depends on X86_LOCAL_APIC && X86_64 && (IRQ_REMAP || HYPERVISOR_GUEST)

arch/x86/include/asm/cpufeature.h

@@ -409,7 +409,7 @@ extern const char * const x86_bug_flags[NBUGINTS*32];
  * fast paths and boot_cpu_has() otherwise!
  */
 
-#if __GNUC__ >= 4
+#if __GNUC__ >= 4 && defined(CONFIG_X86_FAST_FEATURE_TESTS)
 extern void warn_pre_alternatives(void);
 extern bool __static_cpu_has_safe(u16 bit);