gdb, gdbarch: Enable inferior calls for shadow stack support.

gdb, gdbarch: Enable inferior calls for shadow stack support.

Inferior calls in GDB reset the current PC to the beginning of the function
that is called.  As no call instruction is executed the new return address
needs to be pushed to the shadow stack and the shadow stack pointer needs
to be updated.

This commit adds a new gdbarch method to push an address on the shadow
stack.  The method is used to adapt the function 'call_function_by_hand_dummy'
for inferior call shadow stack support.

Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>