arm/edk2-firware: don't handle obsolete GCC releases

There's no point in handling GCC cross compilers that are before version 5, as
GCC 5 came out in 2017 and is obsolete now.

Change-Id: I650ee44c433f70004d20c0219310ddbd4028addb
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>