powerpc/fpu: use generic fma functions

Tested with build-many-glibcs for powerpc-linux-gnu

This is a non functional change and powerpc libm before/after was
byte invariant as compared below:

| cd /SCRATCH/vgupta/gnu/install-glibc-A-baseline
| for i in `find . -name libm-2.31.9000.so`; do
|   echo $i; diff $i /SCRATCH/vgupta/gnu/install-glibc-C-reduce-scope/$i ;
|   echo $?;
| done

| ./aarch64-linux-gnu/lib64/libm-2.31.9000.so
| 0
| ./arm-linux-gnueabi/lib/libm-2.31.9000.so
| 0
| ./x86_64-linux-gnu/lib64/libm-2.31.9000.so
| 0
| ./arm-linux-gnueabihf/lib/libm-2.31.9000.so
| 0
| ./riscv64-linux-gnu-rv64imac-lp64/lib64/lp64/libm-2.31.9000.so
| 0
| ./riscv64-linux-gnu-rv64imafdc-lp64/lib64/lp64/libm-2.31.9000.so
| 0
| ./powerpc-linux-gnu/lib/libm-2.31.9000.so
| 0
| ./microblaze-linux-gnu/lib/libm-2.31.9000.so
| 0
| ./nios2-linux-gnu/lib/libm-2.31.9000.so
| 0
| ./hppa-linux-gnu/lib/libm-2.31.9000.so
| 0
| ./s390x-linux-gnu/lib64/libm-2.31.9000.so
| 0

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>