tools build: Support compiling C++ source file

Add new rule to compile .cpp file to .o use g++. C++ support is required
for built-in clang and LLVM support.

Linker side support will be introduced by following commits.

Signed-off-by: Wang Nan <wangnan0@huawei.com>
Cc: Alexei Starovoitov <ast@fb.com>
Cc: He Kuang <hekuang@huawei.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Zefan Li <lizefan@huawei.com>
Cc: pi3orama@163.com
Link: http://lkml.kernel.org/r/1474874832-134786-2-git-send-email-wangnan0@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
diff --git a/tools/build/Build.include b/tools/build/Build.include
index 0248938..1dcb95e 100644
--- a/tools/build/Build.include
+++ b/tools/build/Build.include
@@ -90,6 +90,7 @@
 # - per object C flags
 # - BUILD_STR macro to allow '-D"$(variable)"' constructs
 c_flags = -Wp,-MD,$(depfile),-MT,$@ $(CFLAGS) -D"BUILD_STR(s)=\#s" $(CFLAGS_$(basetarget).o) $(CFLAGS_$(obj))
+cxx_flags = -Wp,-MD,$(depfile),-MT,$@ $(CXXFLAGS) -D"BUILD_STR(s)=\#s" $(CXXFLAGS_$(basetarget).o) $(CXXFLAGS_$(obj))
 
 ###
 ## HOSTCC C flags