Removing duplicated hash functions in compiler/cclasses.pas

FPHash was duplicated 4 times for different input types. I've
consolidated it to a single function with 3 proxy-functions which can be
inlined.

I've also modified the hash function itself slightly, it now resembled
the original DJB hash and seems to perform a tiny bit better.

git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@23850 3ad0048d-3df7-0310-abae-a5850022a9f2