gcc-plugins: test plugin support in Kconfig and clean up Makefile

Run scripts/gcc-plugin.sh from Kconfig so that users can enable
GCC_PLUGINS only when the compiler supports building plugins.

Kconfig defines a new symbol, PLUGIN_HOSTCC.  This will contain
the compiler (g++ or gcc) used for building plugins, or empty
if the plugin can not be supported at all.

This allows us to remove all ugly testing in Makefile.gcc-plugins.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Kees Cook <keescook@chromium.org>

scripts/Kconfig.include

@@ -25,3 +25,6 @@ cc-option = $(success,$(CC) -Werror $(1) -E -x c /dev/null -o /dev/null)
 # $(ld-option,<flag>)
 # Return y if the linker supports <flag>, n otherwise
 ld-option = $(success,$(LD) -v $(1))
+
+# gcc version including patch level
+gcc-version := $(shell,$(srctree)/scripts/gcc-version.sh -p $(CC) | sed 's/^0*//')