unix: add Ppoll support for linux

Use a shim for Poll on arm64.

Fixes golang/go#16052.

Change-Id: I929e7a2293561bddb9355bf65f98bc68b91905b2
Reviewed-on: https://go-review.googlesource.com/24062
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
diff --git a/unix/ztypes_linux_ppc64le.go b/unix/ztypes_linux_ppc64le.go
index 9c21ecf..7db4c78 100644
--- a/unix/ztypes_linux_ppc64le.go
+++ b/unix/ztypes_linux_ppc64le.go
@@ -618,6 +618,10 @@
 	POLLNVAL  = 0x20
 )
 
+type Sigset_t struct {
+	X__val [16]uint64
+}
+
 type Termios struct {
 	Iflag  uint32
 	Oflag  uint32