colflow: adjust TestHashRouterComputesDestination for big endian

Also add a helper to `util/system` package to expose whether we're
running on a big endian system. (Go doesn't seem to expose this
information explicitly.)

Release note: None

Author: yuzefovich

pkg/sql/colflow/BUILD.bazel

@@ -134,6 +134,7 @@ go_test(
         "//pkg/util/randutil",
         "//pkg/util/stop",
         "//pkg/util/syncutil",
+        "//pkg/util/system",
         "//pkg/util/timeutil",
         "@com_github_cockroachdb_errors//:errors",
         "@com_github_stretchr_testify//require",

pkg/sql/colflow/routers_test.go

@@ -38,6 +38,7 @@ import (
 	"github.com/cockroachdb/cockroach/pkg/util/mon"
 	"github.com/cockroachdb/cockroach/pkg/util/randutil"
 	"github.com/cockroachdb/cockroach/pkg/util/syncutil"
+	"github.com/cockroachdb/cockroach/pkg/util/system"
 	"github.com/cockroachdb/errors"
 	"github.com/stretchr/testify/require"
 )
@@ -724,6 +725,11 @@ func TestHashRouterComputesDestination(t *testing.T) {
 		valsPushed      = make([]int, numOutputs)
 	)
 
+	if system.BigEndian {
+		// On big endian architectures we expect a different distribution.
+		expectedNumVals = []int{238, 260, 259, 267}
+	}
+
 	outputs := make([]routerOutput, numOutputs)
 	for i := range outputs {
 		// Capture the index.

pkg/util/system/BUILD.bazel

@@ -4,6 +4,7 @@ go_library(
     name = "system",
     srcs = [
         "cache_line.go",
+        "endian.go",
         "num_cpu.go",
     ],
     importpath = "github.com/cockroachdb/cockroach/pkg/util/system",

pkg/util/system/endian.go

@@ -0,0 +1,12 @@
+// Copyright 2025 The Cockroach Authors.
+//
+// Use of this software is governed by the CockroachDB Software License
+// included in the /LICENSE file.
+
+package system
+
+import "encoding/binary"
+
+// BigEndian is true if we're running on a system with big endian architecture
+// like s390x (most architectures we're running on are little endian).
+var BigEndian = binary.NativeEndian.Uint16([]byte{0x12, 0x34}) == uint16(0x1234)