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 Alpha-specific values

 CT_DoArchTupleValues () {

     # The architecture part of the tuple:

     CT_TARGET_ARCH="${CT_ARCH}${CT_ARCH_ALPHA_VARIANT}"

 }