Enable -Wshift-overflow=2 to check for undefined shift behavior

The -Wshift-overflow=2 option enables checks for left bit shifts.
Specifically, the option will warn when the result of a shift will be
placed into a signed integer and overflow the sign bit there, which
results in undefined behavior.

To avoid the warnings from these checks, the left operand of a shift can
be made an unsigned integer by using the U() macro or appending the u
suffix.

Change-Id: I50c67bedab86a9fdb6c87cfdc3e784f01a22d560
Signed-off-by: Justin Chadwell <justin.chadwell@arm.com>

Makefile

@@ -256,9 +256,14 @@ WARNINGS	+=		-Wunused -Wno-unused-parameter	\
 				-Wvla
 
 ifeq ($(findstring clang,$(notdir $(CC))),)
+# not using clang
 WARNINGS	+=		-Wunused-but-set-variable	\
 				-Wmaybe-uninitialized		\
-				-Wpacked-bitfield-compat
+				-Wpacked-bitfield-compat	\
+				-Wshift-overflow=2
+else
+# using clang
+WARNINGS	+=		-Wshift-overflow -Wshift-sign-overflow
 endif
 
 ifneq (${E},0)