HPCC-9992 Implement __PLATFORM__ to query the target platform

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #4994 from ghalliday/issue9992

HPCC-9992 Implement __PLATFORM__ to query the target platform

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>