kbuild: doc: gcc to CC change

In this part of the documentation, $(CC) is meant, but gcc is written.

Signed-off-by: Ivan Davydov <davydoff33@yandex.ru>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Author: idavydoff

Documentation/kbuild/makefiles.rst

@@ -578,7 +578,7 @@ cc-option
   Note: cc-option uses KBUILD_CFLAGS for $(CC) options
 
 cc-option-yn
-  cc-option-yn is used to check if gcc supports a given option
+  cc-option-yn is used to check if $(CC) supports a given option
   and return "y" if supported, otherwise "n".
 
   Example::
@@ -596,7 +596,7 @@ cc-option-yn
   Note: cc-option-yn uses KBUILD_CFLAGS for $(CC) options
 
 cc-disable-warning
-  cc-disable-warning checks if gcc supports a given warning and returns
+  cc-disable-warning checks if $(CC) supports a given warning and returns
   the commandline switch to disable it. This special function is needed,
   because gcc 4.4 and later accept any unknown -Wno-* option and only
   warn about it if there is another warning in the source file.
@@ -606,7 +606,7 @@ cc-disable-warning
     KBUILD_CFLAGS += $(call cc-disable-warning, unused-but-set-variable)
 
   In the above example, -Wno-unused-but-set-variable will be added to
-  KBUILD_CFLAGS only if gcc really accepts it.
+  KBUILD_CFLAGS only if $(CC) really accepts it.
 
 gcc-min-version
   gcc-min-version tests if the value of $(CONFIG_GCC_VERSION) is greater than