arm: add -D_FORTIFY_SOURCE=1 to the default cflags

Add additional compile and runtime buffer overflow detection to
Android. As of today, Android supports additional buffer overflow
detection for the following libc functions:

* memcpy
* memmove
* strcpy
* strcat
* strncpy
* strncat

Support for additional functions will be added to bionic libc in
future code changes.

Please see
    * http://gcc.gnu.org/onlinedocs/gcc/Object-Size-Checking.html
    * http://gcc.gnu.org/ml/gcc-patches/2004-09/msg02055.html

for information on these features.

Change-Id: I1a281a4bac20009f4730dd8d8b798fa107576230