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

arch/x86/Kconfig.cpu

···
 
 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