powerpc: refactor logb{f,l}

powerpc: refactor logb{f,l}

The power7 logb implementation does not show a performance gain on
ISA 2.07+ chips with faster floating-point to GRP instructions
(currently POWER8 and POWER9).

This patch moves the POWER7 implementation to generic one and enables
it for POWER7.  It also add some cleanup to use inline floating-point
number instead of define them using static const.

The performance difference is for POWER9:

  - Without patch:
  "logb": {
   "subnormal": {
    "duration": 4.99202e+09,
    "iterations": 8.83662e+08,
    "max": 75.194,
    "min": 5.501,
    "mean": 5.64925
   },
   "normal": {
    "duration": 4.97063e+09,
    "iterations": 9.97094e+08,
    "max": 46.489,
    "min": 4.956,
    "mean": 4.98512
   }
  }

  - With patch:
  "logb": {
   "subnormal": {
    "duration": 4.97226e+09,
    "iterations": 9.92036e+08,
    "max": 77.209,
    "min": 4.892,
    "mean": 5.01218
   },
   "normal": {
    "duration": 4.96192e+09,
    "iterations": 1.07545e+09,
    "max": 12.361,
    "min": 4.593,
    "mean": 4.61382
   }
  }

The ifunc implementation is also enabled only for powerpc64.

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/power7/fpu/s_logb.c: Move to ...
	* sysdeps/powerpc/fpu/s_logb.c: ... here.  Use inline FP constants.
	* sysdeps/powerpc/power7/fpu/s_logbf.c: Move to ...
	* sysdeps/powerpc/fpu/s_logbf.c: ... here.  Use inline FP constants.
	* sysdeps/powerpc/power7/fpu/s_logbl.c: Move to ...
	* sysdeps/powerpc/fpu/s_logbl.c: ... here.  Use inline FP constants.
	* sysdeps/powerpc/powerpc32/power4/fpu/multiarch/s_logb-power7.c:
	Adjust implementation path.
	* sysdeps/powerpc/powerpc32/power4/fpu/multiarch/s_logbf-power7.c:
	Adjust implementation path.
	* sysdeps/powerpc/powerpc32/power4/fpu/multiarch/s_logbl-power7.c:
	Adjust implementation path.
	* sysdeps/powerpc/powerpc64/be/fpu/multiarch/Makefile
	(libm-sysdep_routines): Add s_log* objects.
	(CFLAGS-s_logbf-power7.c, CFLAGS-s_logbl-power7.c,
	CFLAGS-s_logb-power7.c): New fule.
	* sysdeps/powerpc/powerpc64/fpu/multiarch/s_logb-power7.c: Move
	to ...
	* sysdeps/powerpc/powerpc64/be/fpu/multiarch/s_logb-power7.c:
	... here.
	* sysdeps/powerpc/powerpc64/fpu/multiarch/s_logb-ppc64.c: Move
	to ...
	* sysdeps/powerpc/powerpc64/be/fpu/multiarch/s_logb-ppc64.c:
	... here.
	* sysdeps/powerpc/powerpc64/fpu/multiarch/s_logb.c: Move to ...
	* sysdeps/powerpc/powerpc64/be/fpu/multiarch/s_logb.c: ... here.
	* sysdeps/powerpc/powerpc64/fpu/multiarch/s_logbf-power7.c: Move
	to ...
	* sysdeps/powerpc/powerpc64/be/fpu/multiarch/s_logbf-power7.c:
	... here.
	* sysdeps/powerpc/powerpc64/fpu/multiarch/s_logbf-ppc64.c: Move
	to ...
	* sysdeps/powerpc/powerpc64/be/fpu/multiarch/s_logbf-ppc64.c:
	... here.
	* sysdeps/powerpc/powerpc64/fpu/multiarch/s_logbf.c: Move to ...
	* sysdeps/powerpc/powerpc64/be/fpu/multiarch/s_logbf.c: ... here.
	* sysdeps/powerpc/powerpc64/fpu/multiarch/s_logbl-power7.c: Move
	to ...
	* sysdeps/powerpc/powerpc64/be/fpu/multiarch/s_logbl-power7.c:
	... here.
	* sysdeps/powerpc/powerpc64/fpu/multiarch/s_logbl-ppc64.c: Move
	to ...
	* sysdeps/powerpc/powerpc64/be/fpu/multiarch/s_logbl-ppc64.c:
	... here.
	* sysdeps/powerpc/powerpc64/fpu/multiarch/s_logbl.c: Move to ...
	* sysdeps/powerpc/powerpc64/be/fpu/multiarch/s_logbl.c: ... here.
	* sysdeps/powerpc/powerpc64/fpu/multiarch/Makefile: Remove file.
	* sysdeps/powerpc/powerpc64/power7/fpu/s_logb.c: Remove file.
	* sysdeps/powerpc/powerpc64/power7/fpu/s_logbf.c: Likewise.
	* sysdeps/powerpc/powerpc64/power7/fpu/s_logbl.c: Likewise.

Reviewed-by: Gabriel F. T. Gomes <gabrielftg@linux.ibm.com>