Migrate to msp430-gcc-opensource

[jovanbulck]

Currently the Sancus compiler is based on LLVM/Clang, but is meant to be binary compatible with the GNU-based msp430-gcc tool chain. After compilation, msp430-gcc is furthermore used in sancus-ld as the assembler and to link the final program. However, the msp430-gcc tool chain is officially deprecated and succeeded by an official TI gcc port. As such, we are stuck with outdated GCC v4.6 and GNU binutils 2.21 versions.

We therefore should investigate a migration to the upstream TI GCC port. A few remarks and complications:

• Unfortunately, TI GCC introduced major ABI changes; including calling conventions. We thus have to update at least the compiler-generated asm stubs in the LLVM pass and sm_entry/exit stubs.
• For now, we could also install both MSPGCC (msp430-x) and TI GCC (msp430-elf-x) alongside each other, so as to be able to use the more recent GNU binutils in the linker.

[jovanbulck]

Looks like upstream LLVM has already adopted TI GCC EABI calling conventions:

https://reviews.llvm.org/D29069