Enable NX protections

Add -Wa,--noexecstack and -Wl,-z,noexecstack as default
flags when compiling host-side applications.  This enables
NX protections, which prevent code from executing on the
stack or heap.  NX protections make exploiting memory
corruption issues more challenging and is an important
security feature.

Change-Id: Iae580abe887e01f9029ec2a4e0fc0aae496724a4