build: use Ubuntu 12.04 for linux gitian build
gitian: add explicit dependency build for linux

Create a dependency file per architecture:

- bitcoin-deps-linux32-gitian-r1.tar.gz
- bitcoin-deps-linux64-gitian-r1.tar.gz
Merge pull request #3536

0425715 gitian: add explicit dependency build for linux (Wladimir J. van der Laan)
279af1a build: use Ubuntu 12.04 for linux gitian build (Wladimir J. van der Laan)