runtime: use monotonic clock for timers (linux/386, linux/amd64)

runtime: use monotonic clock for timers (linux/386, linux/amd64)

This lays the groundwork for making Go robust when the system's
calendar time jumps around. All input values to the runtimeTimer
struct now use the runtime clock as a common reference point.
This affects net.Conn.Set[Read|Write]Deadline(), time.Sleep(),
time.Timer, etc. Under normal conditions, behavior is unchanged.

Each platform and architecture's implementation of runtime·nanotime()
should be modified to use a monotonic system clock when possible.

Platforms/architectures modified and tested with monotonic clock:
  linux/x86     - clock_gettime(CLOCK_MONOTONIC)

Update #6007

LGTM=dvyukov, rsc
R=golang-codereviews, dvyukov, alex.brainman, stephen.gutekanst, dave, rsc, mikioh.mikioh
CC=golang-codereviews
https://golang.org/cl/53010043