runtime: optimistically CAS atomicstatus directly in enter/exitsyscall

runtime: optimistically CAS atomicstatus directly in enter/exitsyscall

This change steals the performance trick from the coro implementation to
try to do the CAS directly first before calling into casgstatus, a much
more heavyweight function. We have to be careful about synctest
bubbling, but overall it's a good bit faster, and easy low-hanging
fruit.

goos: linux
goarch: amd64
pkg: internal/runtime/cgobench
cpu: AMD EPYC 7B13
           │ after-2-2.out │            after-3.out             │
           │    sec/op     │   sec/op     vs base               │
CgoCall-64     34.62n ± 1%   30.55n ± 1%  -11.76% (p=0.002 n=6)

Change-Id: Ic38620233b55f58b8a07510666aa18648373e2e7
Reviewed-on: https://go-review.googlesource.com/c/go/+/708596
Auto-Submit: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>