unix: add Sysv shared memory support

Implements proposed API from https://golang.org/issue/46084. I chose
`SysvShmDesc` since it's a clearer name than `SysvShm`. Initially supports Darwin and Linux.

Solaris support has a blocker (https://golang.org/issue/46084#issuecomment-836980018)

For golang/go#46084

Change-Id: Ied0f768a74c448254adc3315348417825a7ec63e
GitHub-Last-Rev: befbd7af6b2f838753ac163ef387cebdb7957467
GitHub-Pull-Request: golang/sys#110
Reviewed-on: https://go-review.googlesource.com/c/sys/+/327830
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
diff --git a/unix/ztypes_linux_mipsle.go b/unix/ztypes_linux_mipsle.go
index b43d8e2..5dd546f 100644
--- a/unix/ztypes_linux_mipsle.go
+++ b/unix/ztypes_linux_mipsle.go
@@ -640,3 +640,31 @@
 const (
 	PIDFD_NONBLOCK = 0x80
 )
+
+type SysvIpcPerm struct {
+	Key  int32
+	Uid  uint32
+	Gid  uint32
+	Cuid uint32
+	Cgid uint32
+	Mode uint32
+	_    [0]uint8
+	Seq  uint16
+	_    uint16
+	_    uint32
+	_    uint32
+}
+type SysvShmDesc struct {
+	Perm       SysvIpcPerm
+	Segsz      uint32
+	Atime      uint32
+	Dtime      uint32
+	Ctime      uint32
+	Cpid       int32
+	Lpid       int32
+	Nattch     uint32
+	Atime_high uint16
+	Dtime_high uint16
+	Ctime_high uint16
+	_          uint16
+}