-mcpu=970 causes wrong code on powerpc-linux-gnuspe-
On powerpc-unknown-linux-gnuspe SPE is used for floating point. -mcpu=970 disables the usage of the SPU unit and enables the "normal" floating point unit. This results either in slow code (kernel floating point emulation) or in SIGILL.
-mcpu=970 could also break powerpc-softfloat-linux-gnu but I can't say for sure.