HOST_linux-x86: Avoid doubly define macro _FORTIFY_SOURCE

While building Android with latest host gcc, it causes the following
build error:

Install: out/host/linux-x86/bin/mkbootimg
host C++: obbtool <= frameworks/base/tools/obbtool/Main.cpp
<command-line>:0:0: error: "_FORTIFY_SOURCE" redefined [-Werror]
<built-in>:0:0: note: this is the location of the previous definition
cc1plus: all warnings being treated as errors

In order to make build system happy, this patch does unset macro
_FORTIFY_SOURCE and then define it in host cflags.

Change-Id: Ic37a90f05081a2c3650b3335bc87e9e895900fb2