unix: unify NsecToTime{spec,val}, fix for times < 1970

All the implementations of NsecToTimespec and NsecToTimeval were the
same other than types. Write a single version that uses
GOARCH/GOOS-specific setTimespec and setTimeval functions to handle the
types.

The logic in NsecToTimespec and NsecToTimeval caused times before 1970
to have a negative usec/nsec. The Linux kernel requires that usec
contain a positive number; for consistency, we do this for both
NsecToTimespec and NsecToTimeval.

Follow CL 30826 which did the same for syscall.

Change-Id: Id6c6f4fef8450251447d1a5b01f35c2a36b5aeb1
Reviewed-on: https://go-review.googlesource.com/73170
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/unix/syscall_linux_mips64x.go b/unix/syscall_linux_mips64x.go
index 438fd36..cdda11a 100644
--- a/unix/syscall_linux_mips64x.go
+++ b/unix/syscall_linux_mips64x.go
@@ -76,17 +76,12 @@
 
 //sys	Utime(path string, buf *Utimbuf) (err error)
 
-func NsecToTimespec(nsec int64) (ts Timespec) {
-	ts.Sec = nsec / 1e9
-	ts.Nsec = nsec % 1e9
-	return
+func setTimespec(sec, nsec int64) Timespec {
+	return Timespec{Sec: sec, Nsec: nsec}
 }
 
-func NsecToTimeval(nsec int64) (tv Timeval) {
-	nsec += 999 // round up to microsecond
-	tv.Sec = nsec / 1e9
-	tv.Usec = nsec % 1e9 / 1e3
-	return
+func setTimeval(sec, usec int64) Timeval {
+	return Timeval{Sec: sec, Usec: usec}
 }
 
 func Pipe(p []int) (err error) {