[GOLD] Tweak keep_text_section_prefix test for PowerPC64 ELFv1

[Cary Coutant]

> Tested powerpc64-linux and x86_64-linux.  OK?
>
>         * testsuite/Makefile.am (keep_text_section_prefix_nm.stdout):
>         Pass --synthetic to nm.
>         * testsuite/Makefile.in: Regenerate.

OK. Thanks!

-cary