Fix gcc-4.7.4 and above led to device booting error

In gcc version updated to 4.7.4, when using standard optimization (-O2)
compile code, our device exception reboot at runtime. When using gcc
4.8.0 and above, device could not even start, and this problem also appeared
on lge mako.

When I check gcc-4.9.3 official manual, I notice that ftree-xxx
optimization may cause very serious problem. I tried to
remove these -ftree-xxx optimization flag when compiling the kernel using
gcc-4.9.3, I found -ftree- vrp led to the device can not start.

gcc-4.9.3 manual is this description for -ftree-vrp:

> Perform Value Range Propagation on trees. This is similar to the constant prop-
agation pass, but instead of values, ranges of values are propagated. This allows
the optimizers to remove unnecessary range checks like array bound checks and
null pointer checks. This is enabled by default at ‘-O2’ and higher. Null pointer
check elimination is only done if ‘-fdelete-null-pointer-checks’ is enabled.

To fix errors, use -fno-tree-vrp when compiling the kernel.

Signed-off-by: Andy Deng <theandy.deng@gmail.com>

Makefile

@@ -602,6 +602,8 @@ endif
 
 KBUILD_CFLAGS   += $(call cc-option, -fno-var-tracking-assignments)
 
+KBUILD_CFLAGS	+= $(call cc-option, -fno-tree-vrp)
+
 ifdef CONFIG_DEBUG_INFO
 KBUILD_CFLAGS	+= -g
 KBUILD_AFLAGS	+= -gdwarf-2