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>

 reporter_name="Yann E. MORIN"

 reporter_url="http://ymorin.is-a-geek.org/projects/crosstool"

 reporter_comment="EXPERIMENTAL MIPS-64 n32 toolchain."