unix: make TimevalToNsec available everywhere

It wasn't in linux/arm.

Tested with:

$ for x in $(go tool dist list ) ; do \
     export GOOS=$(echo $x | cut -d/ -f1); \
     export GOARCH=$(echo $x | cut -d/ -f2); \
     echo "$GOOS; $GOARCH"; go install  .; done

... which all pass, except openbsd/arm which is still broken exactly
how it was broken previously.

Fixes golang/go#14643

Change-Id: Ie7ae861b581b539178de26f15ba3f4bdd0e9b785
Reviewed-on: https://go-review.googlesource.com/21013
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall.go

@@ -68,6 +68,8 @@ func (tv *Timeval) Nano() int64 {
 	return int64(tv.Sec)*1e9 + int64(tv.Usec)*1000
 }
 
+func TimevalToNsec(tv Timeval) int64 { return int64(tv.Sec)*1e9 + int64(tv.Usec)*1e3 }
+
 // use is a no-op, but the compiler cannot see that it is.
 // Calling use(p) ensures that p is kept live until that point.
 //go:noescape