Add 2 new toolchain files for ARM cross compilation:
- LinuxARM32 requires `arm-linux-gnueabihf-gcc/g++`
- LinuxARM64 requires `aarch64-linux-gnu-gcc/g++`
Only the build has been tested so far for the aarch64 build (pending,
need to find and install a 64 bits OS), the binaries were not run. The
32 bits binary has been tested on a RPI running raspbian.
Also not tested building `bitcoin-qt`. The `build-unix.md` instructions
for cross compiling suggest to not install Qt as part of the
dependencies, and this seems to be a rare use case for these targets.
Depends on D3017