Makefile: reuse CC_VERSION_TEXT

I noticed we're invoking $(CC) via $(shell) more than once to check the
version.  Let's reuse the first string captured in $CC_VERSION_TEXT.
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
[masahiro.yamada:
CC_VERSION_TEXT is assigned by = instead of :=, so this $(shell ) is
evaluated multiple times anyway. The number of $(CC) invocations will
be still the same. Replacing 'grep' with the built-in $(findstring )
will give real performance benefit.]
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Makefile

@@ -557,7 +557,13 @@ ifdef building_out_of_srctree
 	{ echo "# this is build directory, ignore it"; echo "*"; } > .gitignore
 endif
 
-ifneq ($(shell $(CC) --version 2>&1 | head -n 1 | grep clang),)
+# The expansion should be delayed until arch/$(SRCARCH)/Makefile is included.
+# Some architectures define CROSS_COMPILE in arch/$(SRCARCH)/Makefile.
+# CC_VERSION_TEXT is referenced from Kconfig (so it needs export),
+# and from include/config/auto.conf.cmd to detect the compiler upgrade.
+CC_VERSION_TEXT = $(shell $(CC) --version 2>/dev/null | head -n 1)
+
+ifneq ($(findstring clang,$(CC_VERSION_TEXT)),)
 ifneq ($(CROSS_COMPILE),)
 CLANG_FLAGS	+= --target=$(notdir $(CROSS_COMPILE:%-=%))
 GCC_TOOLCHAIN_DIR := $(dir $(shell which $(CROSS_COMPILE)elfedit))
@@ -576,12 +582,6 @@ KBUILD_AFLAGS	+= $(CLANG_FLAGS)
 export CLANG_FLAGS
 endif
 
-# The expansion should be delayed until arch/$(SRCARCH)/Makefile is included.
-# Some architectures define CROSS_COMPILE in arch/$(SRCARCH)/Makefile.
-# CC_VERSION_TEXT is referenced from Kconfig (so it needs export),
-# and from include/config/auto.conf.cmd to detect the compiler upgrade.
-CC_VERSION_TEXT = $(shell $(CC) --version 2>/dev/null | head -n 1)
-
 ifdef config-build
 # ===========================================================================
 # *config targets only - make sure prerequisites are updated, and descend