Fix linking of fio executable

Fix linking of fio executable

First of all make sure that $(LDFLAGS) are used during linking so that the
user can set the linker's flags.

Then also make sure that libraries are listed after the objects, so that
GNU ld's --as-needed option will not cause fio linking to fail.

Cc: Robin H. Johnson <robbat2@gentoo.org>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>