Added two cmake cross compile toolchain file.

[shiptux]

This commit includes RISC-V and AArch64 cross-compile support for Prometheus-cpp.

Full steps:

git submodule init
git submodule update

mkdir aarch64_build
cd aarch64_build

# run cmake with cross-compile toolchain
cmake .. -DBUILD_SHARED_LIBS=ON -DENABLE_PUSH=OFF -DENABLE_COMPRESSION=OFF -DCMAKE_TOOLCHAIN_FILE=../cmake/toolchain-aarch64.cmake

# build
cmake --build . --parallel 4

# test
Test should runs on QEMU. WIP