runtime: retype gcControllerState.gcPercent as atomic.Int32

[git-generate]
cd src/runtime
mv export_test.go export.go
GOROOT=$(dirname $(dirname $PWD)) rf '
  add gcControllerState.gcPercent \
// Initialized from GOGC. GOGC=off means no GC. \
gcPercent_ atomic.Int32
  ex {
    import "runtime/internal/atomic"

    var t gcControllerState
    var v, w int32
    var d int32

    t.gcPercent -> t.gcPercent_.Load()
    t.gcPercent = v -> t.gcPercent_.Store(v)
    atomic.Loadint32(&t.gcPercent) -> t.gcPercent_.Load()
    atomic.Storeint32(&t.gcPercent, v) -> t.gcPercent_.Store(v)
    atomic.Xaddint32(&t.gcPercent, d) -> t.gcPercent_.Add(d)
    atomic.Casint32(&t.gcPercent, v, w) -> t.gcPercent_.CompareAndSwap(v, w)
    atomic.Xchgint32(&t.gcPercent, v) -> t.gcPercent_.Swap(v)
  }
  rm gcControllerState.gcPercent
  mv gcControllerState.gcPercent_ gcControllerState.gcPercent
'
mv export.go export_test.go

Change-Id: I1aae34a3f782d096c6b6233bbf7986e67ce9c5f9
Reviewed-on: https://go-review.googlesource.com/c/go/+/357794


Trust: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>