cpumask: Add assign cpu

Standardize an assign_cpu function for cpumasks.

Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
Link: https://lore.kernel.org/r/20240312-fencei-v13-4-4b6bdc2bbf32@rivosinc.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>