But are all lazarus libraries 64-bit aware and works under all platforms?
What about visual controls?
If you find any library or control, that doesn't work under 64-bit, then you need to report it as a bug at the mantis: bugs.freepascal.org
There were some 64-bit compatibility issues, but most of them have been fixed, in the mean time.
Carbon (MacOSX) library is 32-bit only, because of Apple limitations.