toolchain/gcc: Enable --with-float for powerpc

Commit 8d929f4b ("toolchain/gcc: Only enable --with-float when it makes
sense") restricted the --with-float use to only MIPS, ARM and SPARC,
while it seems that powerpc needs it as well.

Fixes the qemu_ppc_virtex_ml507_defconfig build.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>