Since compiler may pass --as-needed to ld by default, link .o file
before .so file in i386/x86-64 tests.
PR ld/19774
* testsuite/ld-i386/i386.exp: Link tmpdir/pr18900.o before
tmpdir/pr18900.so and test --as-needed. Link tmpdir/gotpc1.o
before tmpdir/got1d.so and test --as-needed.
* testsuite/ld-x86-64/x86-64.exp: Link tmpdir/pr18900.o before
tmpdir/pr18900.so and test --as-needed.