GDB, gdbserver: aarch64-linux: Initial Guarded Control Stack support

GDB, gdbserver: aarch64-linux: Initial Guarded Control Stack support

Add the org.gnu.gdb.aarch64.gcs feature with the GCSPR register, and the
org.gnu.gdb.aarch64.gcs.linux feature with "registers" to represent the
Linux kernel ptrace and prctl knobs that enable and lock specific GCS
functionality.

This code supports GCS only in Linux userspace applications, so the
GCSPR that is exposed is the one at EL0.

Also, support for calling inferior functions is enabled by adding an
implementation for the shadow_stack_push gdbarch method.

Testcases gdb.arch/aarch64-gcs.exp and gdb.arch/aarch64-gcs-core.exp are
included to cover the added functionality.

The change in the core_find procedure allows aarch64-gcs-core.exp to
recover the output from the crashed program.  It's needed because the
test program in this testcase prints to stdout the value of the GCSPR
right before crashing, and the testcase needs it to check whether GDB
got it right.