[PATCH v2] Support little-endian PowerPC64 and the ELF v2 ABI

This PR is a follow up to PR 1384, addressing some of its comments/requests:
- Use a patch series w/ usable commits messages; by @kumpera [comment] (https://github.com/mono/mono/pull/1384#issuecomment-61375432)
- use ARCH_MIN_MS_BLOCK_SIZE et al in sgen-archdep.h; by @vargaz [comment] (https://github.com/mono/mono/pull/1384#commitcomment-8412464)

The only comment still to-be addressed is [this one] (https://github.com/mono/mono/pull/1384#commitcomment-8412759) by vargaz, which will require some more thinking and time. 

The new changes haven't been committed in that PR's branch as they don't only 'add' commits, but rather insert earlier commits and change an existing one (not sure how that would/n't reflect in the PR history).

I believe the comments/requests have been addressed and can go through another review in this PR.

I tested w/ current HEAD and it fails to build - it seems due to some commit introduced along this 2 weeks or so - thus probably not worth spinning a build in jenkins, but just the review here.

(excuse the old-school/mailing-list title :)

This code is submitted under MIT license (c) IBM and SUSE.