aarch64: Add tests for Guarded Control Stack

aarch64: Add tests for Guarded Control Stack

These tests validate that GCS tunable works as expected depending
on the GCS markings in the test binaries.

Tests validate both static and dynamically linked binaries.

These new tests are AArch64 specific. Moreover, they are included only
if linker supports the "-z gcs=<value>" option. If built, these tests
will run on systems with and without HWCAP_GCS. In the latter case the
tests will be reported as UNSUPPORTED.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>