eal: fix build on FreeBSD

eal: fix build on FreeBSD

The auxiliary vector read is implemented only for Linux.
It could be done with procstat_getauxv() for FreeBSD.

Since the commit below, the auxiliary vector functions
are compiled for every architectures, including x86
which is tested with FreeBSD.

This patch is moving the Linux implementation in Linux directory,
and adding a fake/empty implementation for FreeBSD.

Fixes: 2ed9bf330709 ("eal: abstract away the auxiliary vector")

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Maxime Coquelin <maxime.coquelin@redhat.com>