don't try to clean unsupported CPU-OS combinations
* greatly enhances clean performance over FSes with limited to no caching (networked file systems (eg. SSHFS), Amiga, etc)
* also an indentation fix

git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@25339 3ad0048d-3df7-0310-abae-a5850022a9f2