Upgrade gitian win32 to Precise 12.04.

mingw upgrade allows more hardening, compiler bug fixes and possibily win64 later.

Rename deps .zip files to be more consistent in revision format.
Remove "msvc" compat triplet name.
LDFLAGS -static-libgcc -static-libstdc++
Upgrade win32 gitian to boost-1.54.0.
Workaround 1.54.0  build bug, upstream #9156
Workaround 1.51.0+ human bug, upstream #7262

Upstream 1.51.0 fixed a multiple reference bug that is problematic in 1.50.0.  The cross-compile patch is by the Fedora package developer and shipped in Fedora.

Please also note the sha256sum -c thing in boost-win32.yml.  devrandom agreed it is a good approach to better ensure the integrity of inputs, at least prior to gitian itself gaining the ability to track and enforce particular input hashes.  If folks have no objections to this sha256sum -c approach, I would like to do a different pull request that does it for all inputs.

All unit tests pass.