Makefile: add gcc flags to prevent some optimization

Add the below flags, they are needed in -O2:

-fno-delete-null-pointer-checks:
    * tells the compiler NOT to assume that null pointer deference does
    not exist.

    * Without this flag, below case cannot be detected:
    a pointer might point to nullsometime during run-time and if there
    is no validation for that pointer, it will cause the program to crash.
    Since we don’t receive an error message saying that a pointer is
    pointing to null, we will have a hard time trying to find the problem.

-fwrapv:
    * tells the compiler that signed overflow always wraps.

    * Without this flag, x + 10 > x will always be true for signed x.
     With the flag, x + 10 > x is not always be true, as the overflow is
     defined for x, and it could wrap.

Tracked-On: #4194
Signed-off-by: Conghui Chen <conghui.chen@intel.com>
Reviewed-by: Yonghua Huang <yonghua.huang@intel.com>

misc/efi-stub/Makefile

@@ -74,6 +74,7 @@ CFLAGS=-I. -I.. -I$(INCDIR)/efi -I$(INCDIR)/efi/$(ARCH) \
 
 CFLAGS += -mno-mmx -mno-sse -mno-sse2 -mno-80387 -mno-fp-ret-in-387
 
+CFLAGS += -fno-delete-null-pointer-checks -fwrapv
 ifeq ($(ARCH),ia32)
 	ifeq ($(HOST),x86_64)
 		CFLAGS += -m32