Permit selectable assembler backend
The name of the assembler binary used to generate object outputs was defined by the preprocessor macro ASMCMD: for native builds, it was "ObjAsm" and for cross-compilation builds, it was "armasm". Extend this so that the name of the assembler can be overridden at run time by the environment variable also called ASMCMD. This permits the tool to generate ELF format object files for use with GCCSDK.