gdbserver: Add optional runtime register set type.

Some register sets can be activated and deactivated by the OS during the runtime of
a process.  One example register is the Intel CET shadow stack pointer.  This patch
adds a new type of register set to handle such cases.  We shouldn't deactivate these
regsets and should not show a warning if the register set is not active but supported
by the kernel.  However, it is safe to deactivate them, if they are unsupported by the
kernel.  To differentiate those scenarios we can use the errno returned by the ptrace
call.

Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Approved-By: Luis Machado <luis.machado@arm.com>