This generic implementation already expects a getdents API which
is Linux specific. It also allows simplify it by assuming
_DIRENT_HAVE_D_RECLEN and _DIRENT_HAVE_D_OFF support.
The readdir are also expanded on each required implementation,
futher fixes and improvements will make parametrize the
implementation more complex.
Checked on x86_64-linux-gnu, i686-linux-gnu, and with a build
for all affected ABIs.
