unix: use Getpgid to emulate Getpgrp on linux

Some Linux architectures (e.g. arm64) don't have the getpgrp syscall,
use getpgid(0) to emulate it.

Update golang/go#10150.

This brings CL 8022 that has been applied to the syscall package to
x/sys.

Change-Id: I24c6d7e8b5b2f075ca4e68b142b2e03ab8a43342
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/8023Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/syscall_linux.go

@@ -810,7 +810,12 @@ func Mount(source string, target string, fstype string, flags uintptr, data stri
 //sys	Fsync(fd int) (err error)
 //sys	Getdents(fd int, buf []byte) (n int, err error) = SYS_GETDENTS64
 //sysnb	Getpgid(pid int) (pgid int, err error)
-//sysnb	Getpgrp() (pid int)
+
+func Getpgrp() (pid int) {
+	pid, _ = Getpgid(0)
+	return
+}
+
 //sysnb	Getpid() (pid int)
 //sysnb	Getppid() (ppid int)
 //sys	Getpriority(which int, who int) (prio int, err error)

unix/zsyscall_linux_386.go

@@ -542,14 +542,6 @@ func Getpgid(pid int) (pgid int, err error) {
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
-func Getpgrp() (pid int) {
-	r0, _, _ := RawSyscall(SYS_GETPGRP, 0, 0, 0)
-	pid = int(r0)
-	return
-}
-
-// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
-
 func Getpid() (pid int) {
 	r0, _, _ := RawSyscall(SYS_GETPID, 0, 0, 0)
 	pid = int(r0)

unix/zsyscall_linux_amd64.go

@@ -542,14 +542,6 @@ func Getpgid(pid int) (pgid int, err error) {
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
-func Getpgrp() (pid int) {
-	r0, _, _ := RawSyscall(SYS_GETPGRP, 0, 0, 0)
-	pid = int(r0)
-	return
-}
-
-// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
-
 func Getpid() (pid int) {
 	r0, _, _ := RawSyscall(SYS_GETPID, 0, 0, 0)
 	pid = int(r0)

unix/zsyscall_linux_arm.go

@@ -542,14 +542,6 @@ func Getpgid(pid int) (pgid int, err error) {
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
-func Getpgrp() (pid int) {
-	r0, _, _ := RawSyscall(SYS_GETPGRP, 0, 0, 0)
-	pid = int(r0)
-	return
-}
-
-// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
-
 func Getpid() (pid int) {
 	r0, _, _ := RawSyscall(SYS_GETPID, 0, 0, 0)
 	pid = int(r0)