When adding Mprotect to TestMmap in CL 56172, PROT_WRITE was implicitly
assumed to also include PROT_READ. This is not necessarily true on all
OSes and makes TestMmap crash on openbsd/386.

As suggested my Mikio Hara fix it by also setting PROT_READ on the
memory mapping.

Fixes golang/go#21962

Change-Id: Ia789ab8b31f8dec0c3db23cc950ccdd4aab3db05
Reviewed-on: https://go-review.googlesource.com/65230
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>