FreeBSD 8.x and 9.x on both i386 (x86) and amd64 (x86_64) are fully supported, support for 10.x is in progress, one may experience problems.
Build instructions for FreeBSD are incorporated in the Building Guide. In particular, here is how to set up your FreeBSD system for building GHC.
The recommend way to install GHC on FreeBSD is just to install it as part of the Haskell Platform port, i.e. devel/hs-haskell-platform. This can be done via old pkg_tools:
# pkg_add -r hs-haskell-platform
Or via pkg:
# pkg install hs-haskell-platform
Note that you can find ports for many popular Haskell software on FreeBSD. For the complete listing, please consult the haskell category in the Ports Collection.
An experimental developer repository can be also found on GitHub and you read more about the FreeBSD status on its FreeBSD wiki page.