runtime: print errno on clock_gettime failure on OpenBSD

For #49532.

Change-Id: I5afc64c987f0519903128550a7dac3a0f5e592cf
Reviewed-on: https://go-review.googlesource.com/c/go/+/368334


Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Joel Sing <joel@sing.id.au>
Reviewed-by: Cherry Mui <cherryyz@google.com>