back to more conservative sign extension. this really needs separate handling for ColdFire and normal 68k, because the CF executes 32bit arithmetics only while normal 68k will do any size happily.

git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@27289 3ad0048d-3df7-0310-abae-a5850022a9f2