Follow CL 70590 which did the same for syscall.

unix.Select uses SYS_PSELECT6 on arm64 and mipx64x, however this
syscall expects its 5th argument to be of type Timespec (with seconds
and nanoseconds) instead of type Timeval (with seconds and microseconds)
This leads to the timeout being too short by a factor of 1000.

This CL fixes this by adjusting the timeout argument accordingly,
similarly to how glibc does it for architectures where neither
SYS_SELECT nor SYS__NEWSELECT are available. It also makes Pselect
generaly available on linux.

Updates golang/go#22246

Change-Id: I69f8821a40c59ee469b8a986d784a4db8727ee9a
Reviewed-on: https://go-review.googlesource.com/70610
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>