gcc: don't patch gcc if there are no patches to apply

While the generic package handler checks for a directory with patches
before starting apply-patches.sh, this is not the case for gcc: the
script is called, even if there is no directory with patches. This results
into a build failure, as apply-patches exits with error code 1 if the
directory doesn't exist.

Signed-off-by: Mischa Jonker <mjonker@synopsys.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>