aarch64: Add LD_PRELOAD tests for GCS handling

aarch64: Add LD_PRELOAD tests for GCS handling

There are modeled after the 'shared' tests subset and it adds 4 new
tests:

1. tst-gcs-preload-disabled: checks if GCS is disabled when a LD_PRELOAD
   module is used without GCS marking with default GCS support.

2. tst-gcs-preload-enforced-abort: chekcs if loader aborts startup when
   a LD_PRELOAD is used without GCS marking and GCS is enforced.

3. tst-gcs-preload-optional: checks if GCS is disabled when a LD_PRELOAD
   is used without GCS marking and GCS is optional.

4. tst-gcs-preload-override: checks if GCS is enabled when a LD_PRELOAD
   is used without GCS marking and GCS is overrided.

Checked on aarch64-linux-gnu with Linux 6.18 on Apple M4 emulated (for
BTI support) and on qemu 10.1.50 simulated (for GCS).

Reviewed-by: Yury Khrustalev <yury.khrustalev@arm.com>
Tested-by: Yury Khrustalev <yury.khrustalev@arm.com>