Add bool argument to fetch only one register

Add bool argument to fetch only one register

When trying to fetch the VG register in aarch64_fetch_tdesc_parameter,
remote_target::fetch_registers ends up fetching all registers using the
G packet.  This results in an infinite recursion because GDB will need
to get the VL parameter and thus fetch the VG register again.

To solve the problem, add bool signaling that the target should try to
get just the requested register.

This commit is incomplete because there are some fetch_register
implementations that were not changed to accept the extra argument.

A possibly better and less invasive option would be to add a new
fetch_registers method overload with the new argument to the target_ops
class with a default implementation that would just call the existing
2-arguments version.  This way, only the remote target would need to
override that implementation.  This isn't done here because
make-target-delegates.py doesn't support method overloads and would need
to be adapted.