[PowerPC] Set PROT_READ flag for MF_EXEC to prevent segfaults on PPC machines

The big endian PPC buildbots are all failing now due to calls to cache
invalidation in unit tests on data that has only the PROT_EXEC flag set.
This has been an issue all along on FreeBSD but it can affect Linux machines
depending on configuration.

This patch mitigates the issue the same way it is mitigated on FreeBSD.

Since this is needed to bring the buildbots back to green, I plan to commit this
and allow for post-commit review, but I thought I would also post it here for
ease of access/readability.

Differential revision: https://reviews.llvm.org/D62741

llvm-svn: 362412