[MSP430] Provide a toolchain description

This is an initial implementation for msp430 toolchain including
-mmcu option support
-mhwmult options support
-integrated-as by default

The toolchain uses msp430-elf-as as a linker and supports msp430-gcc toolchain tree.

Patch by Kristina Bessonova!

Differential Revision: https://reviews.llvm.org/D56658

llvm-svn: 351228