First cut at support for building assembly-language binaries

- Caveats:
    - Only GCC/as supported so far
    - Sample only tested on OSX/Intel/x64
    - Significant copy/paste from CPP/C support
    - 'cpp' plugin creates a single 'asm' sourceset, which is not generally useful
Run the assembler once per input .asm file

- This mimics what GCC does, and produces a separate object file per source file
Renamed AssembleStaticLibrary task to CreateStaticLibrary

This makes it more clearly different from Assemble task which drives the assembler
Added a more sophisticated sample for asm, and sample test

- ASM code still only valid for osx/intel
Updated the asm sample so that it should work on Linux as well