[libcxx] Add list of currently known buildbots to libc++ homepage

Summary:
Instead of having to update test results for different platforms it would be helpful to just supply links to buildbots that build+test libcxx.
For the FreeBSD bots Pawel Worach gave me the OK to link to them.

Reviewers: mclow.lists, danalbert

Reviewed By: danalbert

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D4947

llvm-svn: 215882