iozone: fixup build system

Fixup iozone's build system to make it somewhat more flexible.
First make our own linux targets for non-AIO toolchains (uClibc) and
non-threading.
And second, just ditch forced largefile support and expect it from
CFLAGS.
Otherwise using the generic target caused issues like:
http://autobuild.buildroot.net/results/19f606f2978dc3c8039b1bccfcc562b4050083c7/


...which is fixed by this.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>