[zx] Update zxInfoThread for upcoming info structure change.

An upcoming CL[1] is adding additional fields to the zxInfoThread
(zx_info_thread) structure. Update the internal go structures to match.

This needs to be added prior to [1] landing to avoid syscall errors from
Zircon due to the destination zxInfoThread structure being too small.
Having a too-large structure is fine.

[1]: Fuchsia CL I06ecd17add266ba44a1fd2aa0805c9c49e112807

Change-Id: I351156341fa97a967cebdb27e2bf77e0d5730c3e
diff --git a/src/runtime/os_fuchsia.go b/src/runtime/os_fuchsia.go
index bc5be88..0d3ff4d 100644
--- a/src/runtime/os_fuchsia.go
+++ b/src/runtime/os_fuchsia.go
@@ -361,9 +361,14 @@
 	profStkFail = []byte("runtime: failed to allocate stack for the new OS thread\n")
 )
 
+type zxCpuSet struct {
+	mask [16]uint32
+}
+
 type zxInfoThread struct {
 	state                 uint32
 	waitExceptionPortType uint32
+	affinityMask          zxCpuSet
 }
 
 //go:noescape