Aarch64 - Disable executable stacks

[jim-saxman]

Summary: This patch marks assembly files as having a non-executable stack. It also updates the Aarch64 gold linker's command line to force the stack to be non-execuatable and throw a warning if the stack becomes executable in the future.

There are no new unit test case regressions with this patch, and the time to run the unit test suite is greatly reduced.

[jim-saxman]

Hi @mxw, @swalk-cavium, can you please review?

[swalk-cavium]

@jim-saxman - LGTM