Compiling HPCC on ARM

These are the first steps to run HPCC on ARM targets. 

The first patch adds architecture selection, and attempts to make each architecture macro unique, so that testing for each one of them will assume all others are not defined.

The second, uses those macros to separate compilation of x86 code from ARM, so that the compilation actually succeeds on ARM (tested on a Chromebook). It will not run, obviously.

Further changes in the pre-processor will be needed to identify all ARM-specific behaviour (flagged by macro ARMFIX), so that progress can be made slowly towards supporting ARM targets.