arch/powerpc: fix tuple for uClibc

Changeset #7c288c777455 broke the tuple for uClibc-based
powerpc toolchains, by unconditionally forcing CT_TARGET_SYS
to "gnu".

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 # Compute sparc-specific values

 CT_DoArchTupleValues() {

     # That's the only thing to override

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

         CT_TARGET_ARCH="${CT_ARCH}64"

     fi

 }