linux-phytec-fsl

powerpc: Only use -mtraceback=no, -mno-string and -msoft-float if toolchain supports it

These options are not recognised on LLVM, so use call cc-option to check
for support.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>