Print additional information on capability store errors

Print additional information on capability store errors

Augment the warnings about not being able to write to capability registers
or write capabilities to memory. Show the command that needs to be entered
in the shell to enable this particular mode.

gdb/ChangeLog:

2021-06-24  Luis Machado  <luis.machado@arm.com>

	* aarch64-linux-nat.c (store_cregs_to_thread)
	(aarch64_linux_nat_target::write_capability): Update error message.

gdbserver/ChangeLog:

2021-06-24  Luis Machado  <luis.machado@arm.com>

	* linux-aarch64-low.cc (aarch64_target::qxfer_capability): Update
	error message.