sync: fix go1.26 build constraints

[copybara-service]

sync: fix go1.26 build constraints

Fix compile error on go1.26rc1.

go env output

  AR='/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ar'
  CGO_CFLAGS='-O2 -g'
  CGO_CPPFLAGS=''
  CGO_CXXFLAGS='-O2 -g'
  CGO_ENABLED='1'
  CGO_FFLAGS='-O2 -g'
  CGO_LDFLAGS='-O2 -g'
  CXX='/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang++'
  GCCGO='gccgo'
  GO111MODULE=''
  GOARCH='arm64'
  GOARM64='v8.0'
  GOAUTH='netrc'
  GOBIN=''
  GOCACHE='/Users/zchee/.cache/go/go-build'
  GOCACHEPROG=''
  GODEBUG=''
  GOENV='/Users/zchee/.config/go/env/env'
  GOEXE=''
  GOEXPERIMENT='loopvar,newinliner,jsonv2,greenteagc'
  GOFIPS140='latest'
  GOFLAGS=''
  GOGCCFLAGS='-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/dp/89wwdp754235m6jmdzj7p5lm0000gn/T/go-build365494810=/tmp/go-build -gno-record-gcc-switches -fno-common'
  GOHOSTARCH='arm64'
  GOHOSTOS='darwin'
  GOINSECURE=''
  GOMOD='/Users/zchee/go/src/gvisor.dev/gvisor/go.mod'
  GOMODCACHE='/Users/zchee/go/pkg/mod'
  GONOPROXY=''
  GONOSUMDB=''
  GOOS='darwin'
  GOPATH='/Users/zchee/go'
  GOPRIVATE=''
  GOPROXY='https://proxy.golang.org,direct'
  GOROOT='/Users/zchee/sdk/go1.26rc1'
  GOSUMDB='sum.golang.org'
  GOTELEMETRY='on'
  GOTELEMETRYDIR='/Users/zchee/Library/Application Support/go/telemetry'
  GOTMPDIR=''
  GOTOOLCHAIN='go1.26rc1+auto'
  GOTOOLDIR='/Users/zchee/sdk/go1.26rc1/pkg/tool/darwin_arm64'
  GOVCS=''
  GOVERSION='go1.26rc1'
  GOWORK=''
  PKG_CONFIG='pkgconf'

$ git rev-parse --abbrev-ref HEAD
go
$ git rev-parse HEAD
9f2192bea5ab6eeec7cf59c2a0f609fffbadbcca

# my local machine is darwin/arm64

$ GOOS=linux GOARCH=amd64 go1.26rc1 build -o /dev/null -v ./sync
gvisor.dev/gvisor/pkg/sync
# gvisor.dev/gvisor/pkg/sync
sync/runtime_constants_go126.go:22:2: WaitReasonSelect redeclared in this block
	sync/runtime_constants_go125.go:22:2: other declaration of WaitReasonSelect
sync/runtime_constants_go126.go:23:2: WaitReasonChanReceive redeclared in this block
	sync/runtime_constants_go125.go:23:2: other declaration of WaitReasonChanReceive
sync/runtime_constants_go126.go:24:2: WaitReasonSemacquire redeclared in this block
	sync/runtime_constants_go125.go:24:2: other declaration of WaitReasonSemacquire

FUTURE_COPYBARA_INTEGRATE_REVIEW=#12397 from zchee:fix-go126-build-constraints ecdf845