runtime: use atomic C types with atomic C functions

runtime: use atomic C types with atomic C functions

Mark types as _Atomic - fixes breakage introduced in CL 726964
across most LLVM based platforms/builders.

Change-Id: I5e64b9ccb0cf5244977a787a52ee124bc03c10de
Reviewed-on: https://go-review.googlesource.com/c/go/+/728040
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>