aarch64: Add glibc.cpu.aarch64_gcs tunable

aarch64: Add glibc.cpu.aarch64_gcs tunable

This tunable controls Guarded Control Stack (GCS) for the process.

0 = disabled: do not enable GCS
1 = enforced: check markings and fail if any binary is not marked
2 = optional: check markings but keep GCS off if a binary is unmarked
3 = override: enable GCS, markings are ignored

By default it is 0, so GCS is disabled, value 1 will enable GCS.

The status is stored into GL(dl_aarch64_gcs) early and only applied
later, since enabling GCS is tricky: it must happen on a top level
stack frame. Using GL instead of GLRO because it may need updates
depending on loaded libraries that happen after readonly protection
is applied, however library marking based GCS setting is not yet
implemented.

Describe new tunable in the manual.

Co-authored-by: Yury Khrustalev <yury.khrustalev@arm.com>
Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>