GCC independence

The pros-rs build process (the armv7a-VEXos-eabi target and cargo-pros) depends on the Arm GNU toolchain (arm-none-eabi-gcc) for linking and transforming output ELF executables produced by Rust into binaries that can be run on a brain. This dependency is less than ideal because the Arm GNU toolchain is BIG. Uncompressed, it's about 1.7GiB which is pretty abysmal. Ideally we would be using only LLVM binutils and cargo-binutils as most users will already have these installed if they are using Rust.

Attempts have been made at using only llvm for linking, stripping, and objcopying (see PR #84). Ultimately all attempts have failed because of a libgcc dependent unwinding implementation in PROS and strange, likely linker related, memory access violation issues in FreeRTOS task switching code. It could still technically be done if we manually patched libpros' unwinding implementation to be independent of libgcc and figured out what what caused the memory access violations, however this would be difficult and would require a huge amount of testing.

If you are up to the the task of getting a pros-rs project running on a brain without arm-none-eabi-gcc or libgcc, please take a shot at it. Getting this working would be amazing.

TLDR

In order to not depend on GCC or libgcc we need to:

• patch libpros with a custom unwinding solution
• get programs linked with LLVM tools to not segfault
• strip everything related to libgcc from libpros
• update the V5 linker scripts to work with LLVM

PR #84 does a couple of these things, so check it out if you want to try getting this working.