ADSP: Improve ASM support

Enhances CMake support for compiling assembly language on ADI's SHARC and Blackfin DSPs.

• Adds Platform module for finding and testing ADSP compiler (Edit: not required)
• Fixes bug whereby processor architecture was not set from CMAKE_SYSTEM_PROCESSOR with ASM language
• Allows *.s files as assembler sources, following usual platform convention
• Uses .doj as object file extension for consistency with C/CXX output and vendor documentation

Revises !8869 (merged).