autotools.mk: fixes for patch_libtool on host builds

This fixes two issues with the patch_libtool fixup when used by a host
build.

1) The '@' in $(patch_libtool) is a shell syntax error if patch_libtool
is not the first fixup, so I think should be removed regardless of (2)

2) The conditionals for Hooks/HostConfigure/Pre were causing
patch_libtool to be applied twice for host builds. I don't see why the
logic for patch_libtool should be different than the other fixups. Note
that HOST_FIXUP is assigned ?= $(PKG_FIXUP) earlier in the makefile.

Signed-off-by: Andy Leiserson <andy@leiserson.org>
Signed-off-by: Dirk Neukirchen <dirkneukirchen@web.de>