Use the "spinbit" mutex implementation always (including on platforms
that need to emulate atomic.Xchg8), and delete the prior "tristate"
implementations.

The exception is GOARCH=wasm, where the Go runtime does not use multiple
threads.

For #68578

Change-Id: Ifc29bbfa05071d776c23a19ae185891a03a82417
Reviewed-on: https://go-review.googlesource.com/c/go/+/658456


Auto-Submit: Rhys Hiltner <rhys.hiltner@gmail.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>