uk/arch: Make `ukarch_dec` use `ukarch_fetch_sub`

uk/arch: Make `ukarch_dec` use `ukarch_fetch_sub`

Change the `ukarch_dec` definition so that it uses `ukarch_fetch_sub`,
being consistent with the `ukarch_inc` implementation.

Signed-off-by: Razvan Virtan <virtanrazvan@gmail.com>
Reviewed-by: Eduard Vintilă <eduard.vintila47@gmail.com>
Reviewed-by: Marc Rittinghaus <marc.rittinghaus@unikraft.io>
Approved-by: Michalis Pappas <michalis@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #864