[docs] Increase minimum supported GCC version for building LLVM to 4.8

Summary:
The RFC proposal sent to increase the minimum required GCC version
to 4.8 received a lot of support. See the following thread:
http://lists.llvm.org/pipermail/llvm-dev/2016-October/105955.html,

This patch implements that by updating the docs. I believe the
references to libstdc++ 4.7 issues can be removed as well, please
let me know if that is not the case or if they should be updated
a different way.

Reviewers: rengolin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D25683

llvm-svn: 284497