gcc: bump 4.9.x series to version 4.9.3

Drop 110-pr64896.patch and 920-libgcc-remove-unistd-header.patch since
they're upstream.

Tweak 850-libstdcxx-uclibc-c99.patch for this new release.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>