gcc-plugins: allow to enable GCC_PLUGINS for COMPILE_TEST

Now that the compiler's plugin support is checked in Kconfig,
all{yes,mod}config will not be bothered.

Remove 'depends on !COMPILE_TEST' for GCC_PLUGINS.

'depends on !COMPILE_TEST' for the following three are still kept:
  GCC_PLUGIN_CYC_COMPLEXITY
  GCC_PLUGIN_STRUCTLEAK_VERBOSE
  GCC_PLUGIN_RANDSTRUCT_PERFORMANCE

Kees suggested to do so because the first two are too noisy, and the
last one would reduce the compile test coverage.  I commented the
reasons in arch/Kconfig.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Kees Cook <keescook@chromium.org>

arch/Kconfig

@@ -417,7 +417,6 @@ menuconfig GCC_PLUGINS
 	bool "GCC plugins"
 	depends on HAVE_GCC_PLUGINS
 	depends on PLUGIN_HOSTCC != ""
-	depends on !COMPILE_TEST
 	help
 	  GCC plugins are loadable modules that provide extra features to the
 	  compiler. They are useful for runtime instrumentation and static analysis.
@@ -427,7 +426,7 @@ menuconfig GCC_PLUGINS
 config GCC_PLUGIN_CYC_COMPLEXITY
 	bool "Compute the cyclomatic complexity of a function" if EXPERT
 	depends on GCC_PLUGINS
-	depends on !COMPILE_TEST
+	depends on !COMPILE_TEST	# too noisy
 	help
 	  The complexity M of a function's control flow graph is defined as:
 	   M = E - N + 2P
@@ -494,7 +493,7 @@ config GCC_PLUGIN_STRUCTLEAK_BYREF_ALL
 config GCC_PLUGIN_STRUCTLEAK_VERBOSE
 	bool "Report forcefully initialized variables"
 	depends on GCC_PLUGIN_STRUCTLEAK
-	depends on !COMPILE_TEST
+	depends on !COMPILE_TEST	# too noisy
 	help
 	  This option will cause a warning to be printed each time the
 	  structleak plugin finds a variable it thinks needs to be
@@ -534,7 +533,7 @@ config GCC_PLUGIN_RANDSTRUCT
 config GCC_PLUGIN_RANDSTRUCT_PERFORMANCE
 	bool "Use cacheline-aware structure randomization"
 	depends on GCC_PLUGIN_RANDSTRUCT
-	depends on !COMPILE_TEST
+	depends on !COMPILE_TEST	# do not reduce test coverage
 	help
 	  If you say Y here, the RANDSTRUCT randomization will make a
 	  best effort at restricting randomization to cacheline-sized