Fix build issues on BSDs in pkg/cpu (#7116)

Also add a cross compile script to test always cross
compilation for some well known platforms and architectures
, we support out of box compilation of these platforms even
if we don't make an official release build.

This script is to avoid regressions in this area when we
add platform dependent code.

buildscripts/cross-compile.sh

@@ -0,0 +1,35 @@
+#!/bin/bash
+
+# Enable tracing if set.
+[ -n "$BASH_XTRACEFD" ] && set -ex
+
+function _init() {
+    ## All binaries are static make sure to disable CGO.
+    export CGO_ENABLED=0
+
+    ## List of architectures and OS to test coss compilation.
+    SUPPORTED_OSARCH="linux/ppc64le linux/arm64 linux/s390x darwin/amd64 freebsd/amd64"
+}
+
+function _build_and_sign() {
+    local osarch=$1
+    IFS=/ read -r -a arr <<<"$osarch"
+    os="${arr[0]}"
+    arch="${arr[1]}"
+    package=$(go list -f '{{.ImportPath}}')
+    printf -- "--> %15s:%s\n" "${osarch}" "${package}"
+
+    # Go build to build the binary.
+    export GOOS=$os
+    export GOARCH=$arch
+    go build -tags kqueue -o /dev/null
+}
+
+function main() {
+    echo "Testing builds for OS/Arch: ${SUPPORTED_OSARCH}"
+    for each_osarch in ${SUPPORTED_OSARCH}; do
+        _build_and_sign "${each_osarch}"
+    done
+}
+
+_init && main "$@"