powerpc: Remove power4 mpa optimization

This patch removes the POWER4 optimized mpa optimization used currently
on all powerpc targets.  In fact for newer chips, GCC generates *worse*
code than generic implementation as below.  One possibilty would to
add ifunc variants for the mpa routines (as x86_64), but it will add
complexity only for older chips (and one would need to check if
power5, power5+, and power6 do benefict from this optimization),
and only for specific implementation (since most used one such
as sin, cos, exp, pow where optimized to avoid calling the slow
multiprecision path).

* POWER9 patched
$ ./testrun.sh benchtests/bench-atan
  "atan": {
   "": {
    "duration": 5.12565e+09,
    "iterations": 1.552e+08,
    "max": 100.552,
    "min": 7.799,
    "mean": 33.0261
   },
   "144bits": {
    "duration": 5.12745e+09,
    "iterations": 825000,
    "max": 7517.17,
    "min": 6186.3,
    "mean": 6215.09
   }
  }
$ ./testrun.sh benchtests/bench-acos
  "acos": {
   "": {
    "duration": 5.21741e+09,
    "iterations": 1.269e+08,
    "max": 191.738,
    "min": 7.931,
    "mean": 41.1144
   },
   "slow": {
    "duration": 5.25999e+09,
    "iterations": 198000,
    "max": 26681.7,
    "min": 26463.6,
    "mean": 26565.6
   }
  }

* POWER9 master
$ ./testrun.sh benchtests/bench-atan
  "atan": {
   "": {
    "duration": 5.12815e+09,
    "iterations": 1.552e+08,
    "max": 134.788,
    "min": 7.803,
    "mean": 33.0422
   },
   "144bits": {
    "duration": 5.1209e+09,
    "iterations": 447000,
    "max": 11615.8,
    "min": 11301.8,
    "mean": 11456.2
   }
  }
$ ./testrun.sh benchtests/bench-acos
  "acos": {
   "": {
    "duration": 5.22272e+09,
    "iterations": 1.269e+08,
    "max": 115.981,
    "min": 7.931,
    "mean": 41.1562
   },
   "slow": {
    "duration": 5.28723e+09,
    "iterations": 96000,
    "max": 55434.1,
    "min": 54820.6,
    "mean": 55075.3
   }
  }

* POWER8 patched
$ taskset -c 16 ./testrun.sh benchtests/bench-acos
  "acos": {
   "": {
    "duration": 5.16398e+09,
    "iterations": 9.99e+07,
    "max": 174.408,
    "min": 8.645,
    "mean": 51.6915
   },
   "slow": {
    "duration": 5.16982e+09,
    "iterations": 96000,
    "max": 54830.5,
    "min": 53703.8,
    "mean": 53852.3
   }
  }
* POWER8 master
$ taskset -c 16 ./testrun.sh benchtests/bench-acos
  "acos": {
   "": {
    "duration": 5.17019e+09,
    "iterations": 9.99e+07,
    "max": 186.127,
    "min": 8.633,
    "mean": 51.7537
   },
   "slow": {
    "duration": 5.34225e+09,
    "iterations": 90000,
    "max": 60353.2,
    "min": 59155.3,
    "mean": 59358.4
   }
  }

* POWER7 patched
$ taskset -c 16 benchtests/bench-asin
  "asin": {
   "": {
    "duration": 5.15559e+09,
    "iterations": 6.5e+07,
    "max": 193.335,
    "min": 12.227,
    "mean": 79.3168
   },
   "slow": {
    "duration": 5.20538e+09,
    "iterations": 80000,
    "max": 65705.2,
    "min": 64299.4,
    "mean": 65067.3
   }
  }
* POWER7 master
$ taskset -c 16 benchtests/bench-asin
  "asin": {
   "": {
    "duration": 5.15446e+09,
    "iterations": 6.5e+07,
    "max": 184.575,
    "min": 12.226,
    "mean": 79.2994
   },
   "slow": {
    "duration": 5.20616e+09,
    "iterations": 80000,
    "max": 65705.1,
    "min": 64336.6,
    "mean": 65076.9
   }
  }

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/power4/fpu/Makefile: Remove file.
	* sysdeps/powerpc/power4/fpu/mpa-arch.h: Likewise.
	* sysdeps/powerpc/power4/fpu/mpa.c: Likewise.

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