Add 2 new toolchain files for ARM cross compilation:
- LinuxARM32 requires arm-linux-gnueabihf-gcc/g++
- LinuxARM64 requires aarch64-linux-gnu-gcc/g++
The 32 bits binary has been tested on a RPI running raspbian.
The 64 bits binary has been tested on a RPI running Mate 18.04.
I did not test 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