unix: add PidfdSendSignal on Linux

This completes the pidfd_* family of syscalls and adds a test to verify that a
child process can be terminated using the new PidfdSendSignal function.

Change-Id: I71f6a1e26518f513731e20afef02030462e9a2c0
Reviewed-on: https://go-review.googlesource.com/c/sys/+/393794
Trust: Matt Layher <mdlayher@gmail.com>
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/unix/ztypes_linux_mipsle.go b/unix/ztypes_linux_mipsle.go
index e4eb217..a1a28cc 100644
--- a/unix/ztypes_linux_mipsle.go
+++ b/unix/ztypes_linux_mipsle.go
@@ -246,6 +246,13 @@
 
 const _C__NSIG = 0x80
 
+type Siginfo struct {
+	Signo int32
+	Code  int32
+	Errno int32
+	_     [116]byte
+}
+
 type Termios struct {
 	Iflag  uint32
 	Oflag  uint32