runtime: fix time base for time.Now()

This function previously returned the atomic time, that isn't affected
by system time changes but also has a time base at some arbitrary time
in the past. This makes sense for baremetal platforms (which typically
don't know the wall time) but it gives surprising results on Linux and
macOS: time.Now() usually returns a time somewhere near the start of
1970.

This commit fixes this by obtaining both time values: the monotonic time
and the wall clock time. This is also how the Go runtime implements the
time.now function.

src/runtime/runtime.go

@@ -68,27 +68,6 @@ func nanotime() int64 {
 	return ticksToNanoseconds(ticks())
 }
 
-// timeOffset is how long the monotonic clock started after the Unix epoch. It
-// should be a positive integer under normal operation or zero when it has not
-// been set.
-var timeOffset int64
-
-//go:linkname now time.now
-func now() (sec int64, nsec int32, mono int64) {
-	mono = nanotime()
-	sec = (mono + timeOffset) / (1000 * 1000 * 1000)
-	nsec = int32((mono + timeOffset) - sec*(1000*1000*1000))
-	return
-}
-
-// AdjustTimeOffset adds the given offset to the built-in time offset. A
-// positive value adds to the time (skipping some time), a negative value moves
-// the clock into the past.
-func AdjustTimeOffset(offset int64) {
-	// TODO: do this atomically?
-	timeOffset += offset
-}
-
 // Copied from the Go runtime source code.
 //go:linkname os_sigpipe os.sigpipe
 func os_sigpipe() {