Auto merge of #10969 - joshtriplett:available-parallelism, r=epage

Switch back to `available_parallelism`

This reverts commit 8345cf5037506b38457483429fb113322c58f668 now that https://github.com/rust-lang/rust/pull/97925 is merged.

Since that
time, there are now multiple calls to get the number of CPUs, to handle
the `-j -NUM` case, so factor out a helper function.