ftrace: Build with CPPFLAGS to get -Qunused-arguments

When building to record the mcount locations the kernel uses
KBUILD_CFLAGS but not KBUILD_CPPFLAGS. This means it lacks
-Qunused-arguments when building with clang, resulting in a lot of
noisy warnings.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>