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_linux_amd64.go

@@ -96,8 +96,6 @@ func NsecToTimespec(nsec int64) (ts Timespec) {
 	return
 }
 
-func TimevalToNsec(tv Timeval) int64 { return int64(tv.Sec)*1e9 + int64(tv.Usec)*1e3 }
-
 func NsecToTimeval(nsec int64) (tv Timeval) {
 	nsec += 999 // round up to microsecond
 	tv.Sec = nsec / 1e9