[monero-project/monero] One build directory per branch/arch. (#4286)

This proposal allows to perform multiple compilation from different branch/arch in
separate directories.
Example:

build
├── GNU_Linux
│   ├── multi-compilation
│   │   └── release
│   └── NanoS-USBHID
│   └── release
└── Msys
└── NanoS-USBHID
└── release
You can view, comment on, or merge this pull request online at:

https://github.com/monero-project/monero/pull/4286

— Commit Summary —

* One build directory per branch/arch.

— File Changes —

M Makefile (96)

— Patch Links —

https://github.com/monero-project/monero/pull/4286.patch
https://github.com/monero-project/monero/pull/4286.diff