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.

pkg/cpu/counter_linux.go

@@ -20,12 +20,8 @@ import (
 	"syscall"
 	"time"
 	"unsafe"
-)
 
-const (
-	// ProcessClock corresponds to the High-resolution per-process
-	// timer from the CPU represented in stdlib as CLOCK_PROCESS_CPUTIME_ID
-	processClock = 2
+	"golang.org/x/sys/unix"
 )
 
 func newCounter() (counter, error) {
@@ -34,7 +30,8 @@ func newCounter() (counter, error) {
 
 func (c counter) now() time.Time {
 	var ts syscall.Timespec
-	syscall.Syscall(syscall.SYS_CLOCK_GETTIME, processClock, uintptr(unsafe.Pointer(&ts)), 0)
+	// Retrieve Per-process CPU-time clock
+	syscall.Syscall(syscall.SYS_CLOCK_GETTIME, unix.CLOCK_PROCESS_CPUTIME_ID, uintptr(unsafe.Pointer(&ts)), 0)
 	sec, nsec := ts.Unix()
 	return time.Unix(sec, nsec)
 }

pkg/cpu/counter_other.go

@@ -1,3 +1,5 @@
+// +build darwin freebsd openbsd netbsd dragonfly windows
+
 /*
  * Minio Cloud Storage, (C) 2019 Minio, Inc.
  *
@@ -17,12 +19,13 @@
 package cpu
 
 import (
-	"errors"
+	"fmt"
+	"runtime"
 	"time"
 )
 
 func newCounter() (counter, error) {
-	return counter{}, errors.New("cpu metrics not implemented for darwin platform")
+	return counter{}, fmt.Errorf("cpu metrics not implemented for %s platform", runtime.GOOS)
 }
 
 func (c counter) now() time.Time {

pkg/cpu/counter_windows.go

@@ -1,30 +0,0 @@
-/*
- * Minio Cloud Storage, (C) 2019 Minio, Inc.
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *     http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-
-package cpu
-
-import (
-	"errors"
-	"time"
-)
-
-func newCounter() (counter, error) {
-	return counter{}, errors.New("cpu metrics not implemented for windows platform")
-}
-
-func (c counter) now() time.Time {
-	return time.Time{}
-}