runtime: convert schedt.npidle to atomic type

runtime: convert schedt.npidle to atomic type

Note that this converts npidle from uint32 to int32 for consistency with
the other count fields in schedt and the type of p.id.

Note that this changes previously unsynchronized operations to
synchronized operations in:
  * handoffp
  * injectglist
  * schedtrace
  * schedEnableUser
  * sync_runtime_canSpin

For #53821.

Change-Id: I36d1b3b4a28131c9d47884fade6bc44439dd6937
Reviewed-on: https://go-review.googlesource.com/c/go/+/419445
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>