[gitian-build] Change output file destinations to separate directories for each platform

Summary:
Our current release process separates output files by platform, which is more organized
than Core's method of outputing all binaries to the same directory. This modifies gitian-build to match
the process that we currently do by hand.

Depends on D4076

Reviewer note: I did not include the manifest files in this diff because I want to have them reviewed separately.

Test Plan:

./bitcoin-abc/contrib/gitian-build.py -c -n -b gitianuser release-packaging
./bitcoin-abc/contrib/gitian-build.py -c -n -b -o l gitianuser release-packaging
./bitcoin-abc/contrib/gitian-build.py -c -n -b -o w gitianuser release-packaging
./bitcoin-abc/contrib/gitian-build.py -c -n -b -o m gitianuser release-packaging
# Verify files are in the proper sub-directories

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4097