x86/kbuild/64: Test for the availability of the -mtune=native compiler flag

Stephen reported this build failure when cross-compiling:

  cc1: error: bad value 'native' for '-march=' switch

Test for the availability of the -march=native flag.

Reported-by: Stephen Rothwell <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Tested-by: Stephen Rothwell <[email protected]> # build test
Cc: Tor Vic <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Juergen Gross <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Cc: Linus Torvalds <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: Ingo Molnar

arch/x86/Kconfig.cpu

@@ -245,10 +245,14 @@ config MATOM
 
 endchoice
 
+config CC_HAS_MARCH_NATIVE
+	# This flag might not be available in cross-compilers:
+	def_bool $(cc-option, -march=native)
+
 config X86_NATIVE_CPU
 	bool "Build and optimize for local/native CPU"
 	depends on X86_64
-	default n
+	depends on CC_HAS_MARCH_NATIVE
 	help
 	  Optimize for the current CPU used to compile the kernel.
 	  Use this option if you intend to build the kernel for your