introduce HASAMIGA define to easily identify various Amiga flavors (similarly to HASUNIX)

git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@26123 3ad0048d-3df7-0310-abae-a5850022a9f2
use the new HASAMIGA instead of the non-existing AmigaShell

git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@26124 3ad0048d-3df7-0310-abae-a5850022a9f2