x86: also show FS/GS base addresses when dumping registers

x86: also show FS/GS base addresses when dumping registers

Their state may be important to figure the reason for a crash. To not
further grow duplicate code, break out a helper function.

I realize that (ab)using the control register array here may not be
considered the nicest solution, but it seems easier (and less overall
overhead) to do so compared to the alternative of introducing another
helper structure.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Julien Grall <julien.grall@linaro.org>