dev: Fix register bank unit test in .debug

dev: Fix register bank unit test in .debug

The lack of definition of this static variable was throwing
and undefined reference error when running

  scons build/X86/unittests.opt.

For more info, check

  https://stackoverflow.com/questions/
    8016780/undefined-reference-to-static-constexpr-char

Change-Id: Id736dc42a82aea871da6a53c06d89fd399d3559a
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40836
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>