Support cross-compilation

[gavv]

Allow cross-compiling project similar to how it's done here:

• https://github.com/roc-streaming/roc-pulse#cross-compilation
• https://github.com/roc-streaming/roc-pulse#prebuilt-toolchains

In particular:

• Copy setup_toolchain.cmake script that sets all needed cmake vars and supports TOOLCHAIN_PREFIX flag for simple cases: https://github.com/roc-streaming/roc-pulse/blob/main/cmake/setup_toolchain.cmake

• Copy/adjust corresponding sections to README linked above, including examples of compiling for arm32 and arm64 using docker.

• In Makefile, use "build/$(uname -m)" instead of just "build" as build directory. In README examples for cross-compilation, use "build/<toolchain_name>". See here: https://github.com/gavv/signal-estimator/blob/main/Makefile

• In CMakeLists.txt, use "bin/${CMAKE_C_COMPILER_TARGET}" instead of just "bin" to install binaries. See here: https://github.com/gavv/signal-estimator/blob/c09f02382a0afc1edffee8d8eb2f873c44110c34/CMakeLists.txt#L289

• Ensure that at least cross-compiling to arm32 and arm64 using rocstreaming/toolchain-aarch64-linux-gnu and rocstreaming/toolchain-arm-linux-gnueabihf works. If needed, fix building of googletest and roc.

• Add arm32 and arm64 to github actions.