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>

 # Alpha specific configuration file

 ## select ARCH_SUPPORTS_32

 ## select ARCH_DEFAULT_32

 ## select ARCH_USE_MMU

 ## select ARCH_SUPPORT_CPU

 ## select ARCH_SUPPORT_TUNE

 ##

 ## help The Alpha architecture.