gcc: apply PowerPC patch when needed

When converting gcc to the package infrastructure, a specific thing
for PowerPC was forgotten: applying the conditional patch
powerpc-link-with-math-lib.patch.conditional. This breaks the build of
some PowerPC toolchains, with failures such as:

<http://jenkins.free-electrons.com/job/buildroot/./config=qemu_ppc_virtex_ml507_defconfig/ws/output/host/usr/lib/gcc/powerpc-buildroot-linux-uclibc/4.7.3/../../../../powerpc-buildroot-linux-uclibc/lib/libgcc_s.so.1

>: undefined reference to `copysignl'

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Tested-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>