package/rpm: needs gcc >= 5.x on SuperH

Compilation triggers an ICE in gcc with gcc 4.9

../db/dist/../lock/lock_deadlock.c: In function '__lock_detect_rpmdb':
../db/dist/../lock/lock_deadlock.c:354:1: internal compiler error: Segmentation fault
 }
 ^

using this defconfig

BR2_sh=y
BR2_TOOLCHAIN_BUILDROOT_WCHAR=y
BR2_PACKAGE_BUSYBOX_SHOW_OTHERS=y
BR2_PACKAGE_RPM=y

Compiling rpm with gcc5 works fine using this defconfig:

BR2_sh=y
BR2_TOOLCHAIN_BUILDROOT_WCHAR=y
BR2_GCC_VERSION_5_X=y
BR2_PACKAGE_BUSYBOX_SHOW_OTHERS=y
BR2_PACKAGE_RPM=y

This patch adds a dependency to gcc >= 5.x to fix
http://autobuild.buildroot.net/results/e4b/e4b7705e3e148755ae34d498c860a3b9b915e0b0/



[Peter: simpify kconfig, add comment explaining why]
Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>