clang::driver::tools::getCPUName can now infer the CPU being used by
examining either the MCU specified with the -mmcu= option, or the CPU
directly specified with -mcpu=.
This means Clang will now pass the correct "-target-cpu" to downstream
tools, enabling CPU-specific features in LLVM.
The MSP430X CPU can now be selected, in addition to the "generic" MSP430