powerpc: Use generic e_expf

Generic implementation is faster on both power8 and power9:

POWER9:
- sysdeps/ieee754/flt-32/e_expf.c
  "expf": {
   "workload-spec2017.wrf": {
    "duration": 5.1236e+09,
    "iterations": 7.53344e+08,
    "reciprocal-throughput": 5.9436,
    "latency": 7.65869,
    "max-throughput": 1.68248e+08,
    "min-throughput": 1.30571e+08
   }
  }

- sysdeps/powerpc/powerpc64/power8/fpu/e_expf.S
  "expf": {
   "workload-spec2017.wrf": {
    "duration": 5.14429e+09,
    "iterations": 5.29248e+08,
    "reciprocal-throughput": 8.05372,
    "latency": 11.3863,
    "max-throughput": 1.24166e+08,
    "min-throughput": 8.78249e+07
   }
  }

Checked on powerpc-linux-gnu (built without --with-cpu, with
--with-cpu=power4 and with --with-cpu=power5+ and --disable-multi-arch),
powerpc64-linux-gnu (built without --with-cp and with --with-cpu=power5+
and --disable-multi-arch).

	* sysdeps/powerpc/powerpc64/fpu/multiarch/Makefile
	(libm-sysdep_routines): Remove e_expf-power8 and expf-ppc64.
	* sysdeps/powerpc/powerpc64/fpu/multiarch/e_expf-power8.S: Remove
	file.
	* sysdeps/powerpc/powerpc64/fpu/multiarch/e_expf-ppc64.c: Likewise.
	* sysdeps/powerpc/powerpc64/fpu/multiarch/e_expf.c: Likewise.
	* sysdeps/powerpc/powerpc64/fpu/multiarch/w_expf.c: Likewise.
	* sysdeps/powerpc/powerpc64/power8/fpu/e_expf.S: Likewise.
	* sysdeps/powerpc/powerpc64/power8/fpu/w_expf.c: Likewise.

Reviewed-by: Gabriel F. T. Gomes <gabriel@inconstante.eti.br>