x86: remove cc-option-yn test for -mtune=

As noted in the comment, -mtune= has been supported since GCC 3.4. The
minimum required version of GCC to build the kernel (as specified in
Documentation/process/changes.rst) is GCC 4.9.

tune is not immediately expanded. Instead it defines a macro that will
test via cc-option later values for -mtune=. But we can skip the test
whether to use -mtune= vs. -mcpu=.
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

arch/x86/Makefile_32.cpu

@@ -2,13 +2,7 @@
 # CPU tuning section - shared with UML.
 # Must change only cflags-y (or [yn]), not CFLAGS! That makes a difference for UML.
 
-#-mtune exists since gcc 3.4
-HAS_MTUNE	:= $(call cc-option-yn, -mtune=i386)
-ifeq ($(HAS_MTUNE),y)
 tune		= $(call cc-option,-mtune=$(1),$(2))
-else
-tune		= $(call cc-option,-mcpu=$(1),$(2))
-endif
 
 cflags-$(CONFIG_M486SX)		+= -march=i486
 cflags-$(CONFIG_M486)		+= -march=i486