[CMAKE] Add cross build toolchain files for PC Linux platform

Summary:
This allows to use the depends system for building a deterministic linux
binary.

Depends on D3017

Test Plan:
Note: see dependencies in contrib/gitian-descriptors/gitian-linus.yml

mkdir buildcmake && cd buildcmake
cmake -DCMAKE_TOOLCHAIN_FILE=../cmake/platforms/Linux64.cmake \
-GNinja ..
ninja

Run src/bitcoind and src/qt/bitcoin-qt on a 64 bits Linux

rm -rf *
cmake -DCMAKE_TOOLCHAIN_FILE=../cmake/platforms/Linux32.cmake \
-GNinja ..
ninja

Run src/bitcoind and src/qt/bitcoin-qt on a 32 bits Linux

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3019