kbuild: use $(CC_VERSION_TEXT) to evaluate CC_IS_GCC and CC_IS_CLANG

The result of '$(CC) --version | head -n 1' has already been computed
by the top Makefile, and stored in the environment variable,
CC_VERSION_TEXT.

'echo' is cheaper than the two commands $(CC) and 'head' although this
optimization is not noticeable level.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Tested-by: Nathan Chancellor <natechancellor@gmail.com>

init/Kconfig

@@ -9,7 +9,7 @@ config DEFCONFIG_LIST
 	default "arch/$(SRCARCH)/configs/$(KBUILD_DEFCONFIG)"
 
 config CC_IS_GCC
-	def_bool $(success,$(CC) --version | head -n 1 | grep -q gcc)
+	def_bool $(success,echo "$(CC_VERSION_TEXT)" | grep -q gcc)
 
 config GCC_VERSION
 	int
@@ -21,7 +21,7 @@ config LD_VERSION
 	default $(shell,$(LD) --version | $(srctree)/scripts/ld-version.sh)
 
 config CC_IS_CLANG
-	def_bool $(success,$(CC) --version | head -n 1 | grep -q clang)
+	def_bool $(success,echo "$(CC_VERSION_TEXT)" | grep -q clang)
 
 config CLANG_VERSION
 	int