Enable NX protections

Add -Wa,--noexecstack and -Wl,-z,noexecstack as default
flags when compiling applications.  This enables NX
protections, which prevent code from executing on the
stack or heap.  NX protections can block a large number
of buffer overflow attacks, and is an important security
feature.

Change-Id: Iad4bab9f8664584ba6ce832a5318d07680d7a908