aarch64: Add support for GCS in AArch64 linker.

This patch adds support for GCS in AArch64 linker.

This patch implements the following:
1) Defines GNU_PROPERTY_AARCH64_FEATURE_1_GCS bit for GCS in
GNU_PROPERTY_AARCH64_FEATURE_1_AND macro.

2) Adds readelf support to read and print the GNU properties
in AArch64.

Displaying notes found in: .note.gnu.property
[      ]+Owner[        ]+Data size[    ]+Description
  GNU                  0x00000010      NT_GNU_PROPERTY_TYPE_0
      Properties: AArch64 feature: GCS

3) Adds support for -z experimental-gcs linker option and document
all the values allowed with option (-z experimental-gcs[=always|never|implicit]).
-z experimental-gcs is equivalent to -z experimental-gcs=always and
when option is not passed in the command line, it defaults to implicit.

4) Adds support for -z experimental-gcs-report linker option and document
all the values allowed with this option (-z experimental-gcs-report[=none|warning|error]).
-z experimental-gcs-report is equivalent to -z experimental-gcs-report=none
and when option is not passed in the command line, it defaults to none.

The ABI changes adding GNU_PROPERTY_AARCH64_FEATURE_1_GCS to the
GNU property GNU_PROPERTY_AARCH64_FEATURE_1_AND is merged into main and
can be found below.
https://github.com/ARM-software/abi-aa/blob/main/sysvabi64/sysvabi64.rst