base: Narrow the applicability of the default to_number.

base: Narrow the applicability of the default to_number.

That template only works for integral (except bool), floating point,
or enum types, so restrict it to those types. That makes it easier to
detect what types will work with that function.

Change-Id: Ib29a9a0ea75dd617e28bb6850d60be905f93182f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/36279
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>