GDB: aarch64-linux: Implement GCS support in displaced stepping

GDB: aarch64-linux: Implement GCS support in displaced stepping

When doing displaced step on a branch and link instruction with the Guarded
Control Stack enabled, it's necessary to manually push and pop the GCS
entry for the function call since GDB writes a simple branch instruction
rather than a branch and link instruction in the displaced step buffer.

The testcase exercises GCS with displaced stepping by putting the
breakpoint on the bl instruction to force GDB to copy it to the
displaced stepping buffer.  In this situation GDB needs to manually
manage the Guarded Control Stack.

Reviewed-By: Christina Schimpe <christina.schimpe@intel.com>
Approved-By: Luis Machado <luis.machado@arm.com>