Add sancov plugin

The sancov gcc plugin inserts a __sanitizer_cov_trace_pc() call
at the start of basic blocks.

This plugin is a helper plugin for the kcov feature. It supports
all gcc versions with plugin support (from gcc-4.5 on).
It is based on the gcc commit "Add fuzzing coverage support" by Dmitry Vyukov
(https://gcc.gnu.org/viewcvs/gcc?limit_changes=0&view=revision&revision=231296).

Signed-off-by: Emese Revfy <re.emese@gmail.com>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Michal Marek <mmarek@suse.com>

scripts/gcc-plugins/Makefile

@@ -14,8 +14,14 @@ endif
 
 export GCCPLUGINS_DIR HOSTLIBS
 
+ifneq ($(CFLAGS_KCOV), $(SANCOV_PLUGIN))
+  GCC_PLUGIN := $(filter-out $(SANCOV_PLUGIN), $(GCC_PLUGIN))
+endif
+
 $(HOSTLIBS)-y := $(GCC_PLUGIN)
 always := $($(HOSTLIBS)-y)
 
 cyc_complexity_plugin-objs := cyc_complexity_plugin.o
+sancov_plugin-objs := sancov_plugin.o
+
 clean-files += *.so