libunwind: Fix build on ppc

libunwind: Fix build on ppc

gcc driver is fixed to pass correct options to linker to do secure plt
linking when enabled. however this option --secure-plt is not supported
by gold linker which is now passed by default from gcc driver. Hence
the build fails when using gold. Therefore when we use gold then we do
not use secure plt

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>