perf symbols: allow forcing use of cplus_demangle

For Fedora, I want to force perf to link against libiberty.a for
cplus_demangle, rather than libbfd.a for bfd_demangle due to licensing insanity
on binutils. (libiberty is LGPL2, libbfd is GPL3.)

If we just rely on autodetection, we'll end up with libbfd linked against us,
since they're both in binutils-static in the buildroot.

Cc: Ingo Molnar <mingo@elte.hu>
LKML-Reference: <20100510204335.GA7565@bombadil.infradead.org>
Signed-off-by: Kyle McMartin <kyle@redhat.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>