cc/gcc: Add the ability to build gcc from svn

I took some of the svn functionality from eglibc.

Signed-off-by: Bryan Hundven <bryanhundven@gmail.com>
[yann.morin.1998@free.fr: fix the conditional test in build script]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

 # Compute s390-specific values

 CT_DoArchTupleValues() {

     # That's the only thing to override

     if [ "${CT_ARCH_64}" = "y" ]; then

         CT_TARGET_ARCH="s390x"

     fi

 }