pw_kernel: Signal objects with masked SignalUpdate

Every ObjectBase::signal() caller passed a closure that was a pure mask
operation, so each call site paid for its own instantiation of the
update path. Take a SignalUpdate (mask and values) instead.

Code size (bytes):

  mps2_an505         ipc.elf         66348 -> 65768  (-580)
  mps2_an505         wait_group.elf  68388 -> 67848  (-540)
  qemu_virt_riscv32  ipc.elf         68384 -> 67568  (-816)
  qemu_virt_riscv32  wait_group.elf  70648 -> 69868  (-780)

Change-Id: I2c5653105c14968e7ac83afc0ae836ace4ef9431
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/477126