change ifunc assembly test to a link test

The test currently tests the binutils frontend support which passes for
all versions of binutils we currently require (2.20+).  It doesn't test
the backend which is required for ifunc to actually work, and which most
targets don't yet support.

Change the assembly code so that when we link it, we get a file that has
ifunc relocations if the backend supports it.  That way we can test to
see if binutils supports everything we need.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>