Since compiler may pass --as-needed to ld by default, link .o file
before .so file in ifunc tests.
PR ld/19774
* testsuite/ld-ifunc/ifunc.exp: Link tmpdir/pr18808a.o before
tmpdir/libpr18808.so. Link tmpdir/pr18841a.o before
tmpdir/libpr18841b.so and tmpdir/libpr18841c.so. Test
--as-needed for pr18841c.