runtime: clear upper half of sec in ARM walltime

https://golang.org/cl/246763 accidentally changed this to assign the old
value of vdsoPC to the upper half of sec as that is the last value in
R1.

The old value of vdsoPC is usually zero, so this typically works.
However, the reentrant case will have a non-zero value, resulting in a
bogus returned time. I am unsure if walltime is reachable from a signal
handler, so this may never occur.

Change-Id: I8562d6ec2a845fcffee9618d994b8ea57fbd199e
Reviewed-on: https://go-review.googlesource.com/c/go/+/361159


Trust: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>