Use wevtutil.exe to query event log instead of powershell's Get-EventLog
command, which is not available on Windows IoT Core.

Updates golang/go#26148

Change-Id: Idc16d0b18d2826730d2b4062388b2a764c4886c6
Reviewed-on: https://go-review.googlesource.com/c/154817
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>