Make ghc/lib/compat/Compat/Directory.hs use the C function
	__compat_long_path_size, rather than
	__hscore_long_path_size, as the libraries/ version does
And make ghc/lib/compat/cbits/directory.c define it.

In this way we avoid spurious duplicate-symbol errors when we
compile GHC with ghc6.2.1 etc.